I've compiled this list from various sources making some minor changes to the location instructions as I go, because the originals aren't very clear. I've also added links to a photos of each of the locals which is very helpful.
In addition to this I used the Kilroy Alerts code: SH2VYIVNZF
It can be entered from the 'Enter Code' option on the main menu. This will tell you when you're near a Kilroy but can be frustrating at times, because it doesn't specify direction or distance. So it can be on a floor above or below, or in your very general area - that sort of thing.
1/3 - Small shed to the right of the Windmill as you face the stairs up to it.
2/3 - Top floor of the farmhouse in the small room directly opposite the entrance to the bedroom.
3/3 - After clearing the field when you get control of two fire squads, look for the small shack. Look through the window on the North side of the shack and the Killroy will be on the inside adjasent wall.
1/3 - After climbing the windmill you will see a woman being dragged into a barn. Inside that barn, walk upstairs and turn left and procede around to the left all the way to the end of the balcony to find a Kilroy on the wall of the barn's main entrance.
2/3 - Go downstairs inside the manor house, and enter the garden. Continue until you encounter a machine gunner shooting on the ground floor, to your left as you exit the doorway. Turn to the right to find a Kilroy on the wall.
3/3 - After the intermission sequence showing you crossing the river to reach the bridge, look to the left to see two parked cars and a few upturned rowboats. Go to the rowboats, and look at the wall between two open shutters is the hidden Killroy.
Written In Stone
1/3 - Enter the church and after clearing out the Church pues section of Germans you'll head upstairs. There will be another fire fight with Germans across the arcade and before you cross the wooden plank. You'll then continue to cross a wooden plank and the Kilroy will be found in the stained glass alcove, second one to your left down the long hall.
2/3 - Get past the barricade, and complete the "Destroy The German Fuel Supply" objective. You will enter a garden area with some dug in Nazis and a machine gunner in a window on the top floor of a building. Immediately follow the path to the south. When the path turns west, you can find the Kilroy on the wall.
3/3 - Move up the street from the first gun you destroyed as part of an objective. Find the building with green canopies on your right. On the left-hand side and almost directly opposite is a building with black and white striped canopies covering its windows. On the side of this building is a route leading west. Go around to the rear of the building, and jump over the wall. In the next garden is a Kilroy next to a Lager Bier poster.
1/3 - After you are ordered to secure the ground floor of the starting area, go to the hole in the wall where the gun emplacements are located. Move north through the door to enter a narrow flooded area. Go to the end to find a Kilroy on the wall.
2/3 - After you are given the order to clear the top floor of the warehouse, there will be an intermission sequence showing you ascending in an elevator. Afterwards, go to the doorway to the west. Immediately turn right to face north after you enter the doorway. The Kilroy is on the wall.
3/3 - After getting the objective to clear the roof and the intermission sequence ends, kill the sniper on the "PHILLIPS" from a distance. Stand faceing the "PH" in "PHILLIPS" without being on the platform, and turn around 180°. Walk north, staying right of the triangular skylights, until you reach a wall, then turn right 90°. The Kilroy will be in front of you on that very dark wall about chest height. (Note: If you stand on the platform with "PHILLIPS", the storyline will continue and cause this Kilroy to be missed.
Baptism Of Fire
1/3 - Use the front door next to the flames to enter the gas station. Enter the second room to the east. When you enter the small doorway, turn around to face south to find a Kilroy on the wall.
2/3 - Leave the bomb shelter, and enter the area after the long corridor. After the intermission sequences featuring Peter's face, you will enter a ruined area. Go east from your starting point on the lower floor. Go through the archway, and continue ahead until you reach an open room. Move east towards the hole in the east wall in the room. Approach the hole, and turn around to face west. The Kilroy is on the left side of the wall in front of you.
3/3 - After the intermission sequence showing Frankie running off with a woman in white, you will enter a small back alley area on fire with Nazis. Kill the Nazis, and locate the arch in the northern part of this area. Instead of passing through it, go west, and follow the steps going down to find a Kilroy on the wall.
The Rabbit Hole
1/3 - Enter the hospital. Climb through the window, and move east towards the fires. Go through the door next to the fire, and immediately go east past a pair of beds. Go through the door in the south to get to the other side of the fires. Go through the doorway towards the south, opposite from where you entered. A Kilroy is behind the green screen opposite an eye test chart.
2/3 - While searching for Frankie, you will reach a green-lit open room where you must climb through a window to continue. After entering a kitchen take cover behind the two wooden cutting tables, and kill the two Germans that come through the door in front of you. Walk through those doors and turn left into a dead end hallway, just past the columns about midway look to your left on the wall will be the Kilroy.
3/3 - Enter the hole in the wall in the kitchen near where the previous Kilroy was located. After entering the dark space, keep moving north. You might be able to see the Kilroy through a gap in the wall. After the intermission sequence, continue through the dark space, and exit from the hole. Move west, and exit the doorway to the north. Go to the west, and enter the room on your left. The Kilroy is on the north wall.
1/3 - Clear the market square, then go through a building and out into an open area. Find the coffee shop to the left. On the side of the coffee shop is an alley. Move east down the alley, and flank the Nazis. Move towards and approach the wooden barn-like structure. You will find a small shed on your right with Lager/Bier poster. Get over the wall next to the shed. A Kilroy is on the side of the shed.
2/3 - Enter the area past the church. Move towards the municipal building, and enter the area of narrow streets. Find the location where there are buildings with a Dutch flag on your left and right. Enter the building on your left. When you exit, you will see a truck to the south. A Kilroy is on the wall next to that truck.
3/3 - Exit the municipal building, and take a right turn next to the building with the Dutch flags on your right. Move to the east, destroying the first tank, and ignore the second tank. Walk around the triangular-roofed building with a flower pot outside the door at the end. A Kilroy is on the wall next to where the second tank is parked.
1/3 - Move up the road towards the 88mm gun emplacement. There is a Kilroy on the back wall of the building next to the gun. To find it, stand next to the gun before destroying it, and move east, then north and around the back of the building. The Kilroy is next to a trough.
2/3 - Move towards the second 88mm gun emplacement. Move through the barn. When you exit the structure, look to the left to see sandbags and a camo truck. Notice the small building and a few cylindrical-shaped structures near the truck. Search the side of the smaller building to find a Kilroy.
3/3 - Get past the intermission sequence featuring you entering the burning train station house. Reunite with your squad. Climb up the stairs of the water tower where you can climb up the stairs. Reach the top of the first set of stairs to find a Kilroy on the flat part of the bricks under the water tank.
Tooth And Nail (no photos yet)
1/3 - Get past the sand dunes at the start of the mission. Kill the Nazis at the dug in second 88mm gun emplacement. Stand next to the gun emplacement, and turn to the south. You should see a bunker. Enter it from the side you are facing. Look to the left as you enter to find a Kilroy.
2/3 - After the windmill intermission sequence, move east through the first small barn. Go to the large building past it. You will see a few barrels to the right and a truck in front of you. Enter the large barn through the right side of the two double doors. Turn around to find a Kilroy on the west wall.
3/3 - Clear the area around the fifth 88mm gun emplacement. Stand next to the gun emplacement, and turn to the west. Go towards the house with flower pots on the window sills. Move to the right around the side of that house. Find the grey truck on your right. There is a small shed with a Kilroy on the wall.
*Good luck soldier!