If you recall, last year I was remodeling a basement bathroom and discovered somebody had placed a shower stall over a drain pipe with no trap, leading to funky smells in the basement. I broke the concrete out, put a trap in, got the remodeling done, all is well.
Now the upstairs bathroom on the other hand....I noticed when the attic ( whole house ) fan was on recently on a nice evening that the upstairs bathroom had that funky aroma, further, I noticed that the tub seemed to be giving it off....
Now that I think of it, I don't think there is a trap under it...shouldn't it have one?
I know the other plumbing in the house has one, I've replaced them already as they were the old thin chrome metal ( easy to corrode ) traps under the sinks and they collapsed when I was working with them.