I've been curious about a mysterious void between two rooms of my new old house. Boing Boing reader Medievalist explains how to proceed.
To examine a secret room or priest's hole, use a 1.5" or larger hole saw to create an opening you can push a camera and light through. Understand that if you hit a stud you'll probably destroy the hole saw in one use, because you're quite likely to hit multiple nails in the lath. Even if you don't hit any nails you'll still wreck a cheap hole saw because of the sand in the brown coat. Do not just drill until you stop feeling any resistance….