While preparing to mount a bracket for some shelves, I attempted to use a stud finder to locate studs. The device reported four areas as studs, but curiously they did not run vertically. I was expecting to find vertical wooden studs, but these four areas were surrounded by non-stud space on all sides - and the “tap test” confirmed them as solid while all around them sounded hollow.
The four areas were arranged as the corners of a square, about 14 inches on each side. The areas themselves were two or three inches in diameter - hard to assess their exact shape.
I decided to drill into them to investigate - about 2 inches deep. The drill came out covered in white powder, as if I had just drilled through a particularly thick piece of drywall.
Why would there be thick plaster at these points, and no apparent studs in the rest of the wall?