Simple hidden compartment
In my old house I had a hidden gun safe; nobody knew it was there. If you have a setup like I had it is easy.
I put it in the floor of a bedroom, between 2 floor joists. The space I had was between 2 floor joists, about 16 inches wide and 4 feet long, 10 inches high minus the thickness of a board I put over the drywall of the ceiling below )I didn't trust drywall to hold much weight). The space was big enough to store several long guns and other valuables I wanted out of sight.
The hidden part is the most important; if you have a place in your house laid out like this, its easy: wall to wall carpet with a section 18 inches long and 4 feet wide, bordered on 3 sides by walls, with the floor joists running the long direction; if they go the short way it won't work.
If your carpet is not brand new and the area isn't heavily used, you can (gently) separate the carpet from the tack strip around the 3 edges and it won't shrink (undo the stretching when it was installed). You may want to practice undoing carpet from tack strips in a closet or other unobtrusive place. After you have the carpet up, outline an area in the subfloor to cut through; if you can run the cut along the top of a joist you don't have to add reinforcing to support the removable cutout.
Once the panel is put back into place, the carpet rolls back over it and nothing shows.
I had a lip under the subfloor along one side that made it easy to add a lock, so even if someone had found it they couldn't get in easily. Some state require guns to be locked up, and this design would meet such requirements. I don't have any pictures since it was a secret when I made it.
Let me know if you want more information/ descriptions.