I once used a stud finder on the wall of my NYC apartment and found what I thought was a stud. It was above and just off to the side of the outlet, so that made sense to me.

I drilled into it, and it stopped the drill bit. I probed around with a nail and realized that whatever I hit was cylindrical; the nail would slide off around a curve.

It was the conduit for the electrical.

There are no studs in that wall. It's all masonry/plaster brick.

u/zeurydice, I used a hammer drill and Tapcon drill bit and Tapcon screws to mount the shelf standards.