Patty, you might try looking someplace else. Really look closely upstairs for ANY sign of water entry. This would be places like small round discoloration marks on drywall surfaces, disruption of the dirt that inevitably ends up on walls that has moved and collected, heavy patterns of joist or stud locations visible as tracing from the inside of the house.
Then look outside. Look for any sign of peeling or blistered paint on soffitts and eaves, check around chimney and plumbing stack locations, double check if you made any changes like adding a ceiling fan in the bathroom.
Fianlly, look up in the attic for water signs on any of the wood. This will probably be more like discoloration but can be damp spots and they could be anywhere. My guess is that you do have water getting in from on top and finding it's way into a wall or walls. That is what lets the mildew grow but paint barriers or vapor barriers limit the signs from coming through the walls. The smell can "come out the bottom" of your walls, though.
Other things to check are floor drains in the basement. They have "P" traps under them, just like under the sinks in more visible plumbing. If the trap loses water through evaporation or a cracked pipe, this presents an opening for sewer gas to come up. This might be what you are smelling. The "cure" for this is to put about 2 cups of water down each floor drain to refill the traps. If you have to do this every month or so, it's usually no big deal. More often than that can indicate a leaky pipe and this should be repaired by a pro.
That program should get you started. One tip is to carry a note pad with you as you look around and write down your observations immediately. It's easy to forget that little drip mark on the ceiling an hour or so later.