The smell may possibly be coming from your drains. Try putting baking soda down there and then pouring boiling hot war down there that should help eliminate the smell if that is where is coming from. Some people also recommend adding vinegar to that mix.
If you can smell mildew but cannot see it, you have to start looking for it in less obvious places. Perhaps on the underside of surfaces, perhaps behind fixtures or even in fans or vents. There may also be mildew behind drywall. That may take a moisture meter to find.
This is something that is behind the walls. If you have tiles, see if they are loose and moisture is getting behind them. This might be a major renovation. Which means tearing down walls.
Fresheners will normally only mask the smell. Get a rag and a cleaning agent (Barkeeper's friend, etc) and clean out all the areas that get wet in your bathroom.
First things first. If you have the smell, it is somewhere. Mildew is nothing to take lightly, find the source and bleach it.Bleach kills mildew, but it has to be on the source.
