This is a copy of RadiantViper's perfect guide from the One version, posted here with permission since they don't have it on Series X. The pictures will almost certainly not look right, so the link to that guide is here:
Hidden Location 1: At the Crossroads, go behind the Nail structure to find a series of ledges. On one of these ledges is a tunnel with some wooden boards in the entrance. Go through the tunnel for the location, a collectible, and a chest (which spawns enemies).
Hidden Location 2: At the top of the Warehouse elevator (floor 6), go behind the elevator and you can break the wall here to reveal a control point.
Hidden Location 3: Standing at the entrance to the Warehouse tower on level 1, look up and you'll see another entrance above on the same side. You need to float down to this hole from a higher level (try level 5, out the window and around to the other side of the building). Please note that there is an entrance on the same wall higher up - the hidden location is the lower one.
Hidden Location 4: In Collapsed Department, in the multi-level area with lots of office rooms. From where you enter, take a right and go into the lit-up office with a painting. Breaking the wall to reveal this hidden area. There is also a second breakable wall inside that leads to a Maneki-neko.
Hidden Location 5: In Collapsed Department, the same area as Location 4. Go up to the top left side (south on the map) and locate another lit-up office with a painting on the wall. Break the wall and head inside.
Hidden Location 6: In Collapsed Department, near the transit area where the film camera was stored before you move it. On the right side is a small elevator, activate it then step off to get underneath.
Hidden Location 7: In Collapsed Department, after you pass through the room that held the film camera. Take a left and there's a elevator you can open, an enemy pops out. Step inside the elevator for this location.
Hidden Location 8: In the Canyon Rim area there is a passageway with two wooden beams crossing over it. The closest control point is Astral Collision, from there turn right and look down to find it. Grab the golden cube nearby, then turn left around the corner to launch it into one of the bathroom stalls. Float into the other stall for this location. After you do this you can also go to the Astral Bathroom location in the Collapsed Department to find a golden cube and put it in the socket for a Maneki-neko.
Hidden Location 9: In the Canyon Rim area, right beside the Astral Tower PE control point is a raised platform with a TV on it that counts as a hidden location. Upper right in the screenshot.
Hidden Location 10: In the Cave System Area, you will need to progress until the point you unlock the Cave Bridge control point. From here, levitate across to the farthest stone bridge on the east side. Head into the side room here for the location.