While playing on my favorite server, I activated a treasure quest. I had the Treasure Hunter perk maxed out, and the compass indicated that it could be under the Poopy Pants Daycare POI. So I dug down into the asphalt, and started mining around under the area. It soon became obvious that something wasn't right, because it was nowhere to be found in the ground. Ok, so maybe it's on the roof? I checked the roof, but saw nothing. Then I checked inside the POI, but couldn't find it. Not even one of the server admins could figure out where it was. I was about to cancel the quest when he said he wanted to check one more thing.
He found it, inside the roof above the area outside the front door after clipping into the roof. That part of the building has no open spaces, it's all solid blocks. I cut into the roof and found it. So, not only can treasure chests spawn on roofs, but also in walls and other parts of buildings with no open space around it. Skip to 17:36 in the video to see the area I'm talking about, above the drive-thru between the front door and columns.