Using the flashlight (Left on D-Pad), items can be revealed when your beam of light hits them. You’ll see a sparkle and a little glimmer of light on the item.
If you’re looting a house, you should probably have the flashlight on anyway, and you’ll find a lot of these hidden items accidentally. Around light sources there are also hidden items to be revealed.
If you’re going for this achievement, you can probably reveal 20-30 items per game, but you should get this one while playing normally, so save it for last if you’re going for the 100%!