There are two large built-in wooden storage cabinets in the small dressing area that's just off my bedroom in our house. These are shown below.
When I realized there had to be a significant amount of empty space behind the wall above them, I pulled everything out of these cabinets and checked them. Sure enough, the backs of both sides give off radio frequency (RF) signals indicative of the presence of illegal surveillance cameras and microphones installed behind them. And now that I know what to look for, it's easy to spot the telltale large, new, crude screws and scratches and pry marks that show where the wooden backs of the cabinets were pulled open to insert the illegal equipment and then quickly screwed and glued closed. There's electrical wiring for a light switch located nearby that's convenient for tying into, too.
The head of my bed sits up against the other side of this wall and is located directly behind these cabinets.