Plumbing – tracking down sewer gas smell in bedroom

Ever since moving into our new house roughly a month ago, there's been a distinctive (but not overpowering) sewer gas smell in one of our bedrooms. Some information that may be pertinent:

• House was built in the 1920s.
• We had an exterminator come to the house, to rule out that we were mistaking a dead animal smell for a sewer gas smell. Exterminator doesn't think it's a dead animal.
• The smell is truly only in this one bedroom. It's a second floor bedroom, sharing one wall with the master bathroom, one wall with another bedroom, its floor with the dining room, and its ceiling with the attic. The smell is not in the master bathroom, nor in the attic. Occasionally we think we catch a faint whiff elsewhere in the house, but we could easily be imagining that.
• We are able to get into the wall between the smelly bedroom and the master bath (there's an access panel for the whirlpool tub), and as far as we can tell the smell isn't there.
• The strength of the smell is such that you can mostly get rid of it by airing the room out, but if you close the door and windows it comes right back in not too much time.

We initially suspected a problem with the vent pipe (even though that's not supported by the smell being neither in the adjacent bathroom nor the wall). We've talked to a few plumbers and didn't really get anywhere. Plumber #1 refused to do a peppermint test because they didn't want to get anywhere near our sloping slate roof, but had no other ideas. Plumber #2 did pour a couple of bottles of peppermint extract down the master bathroom sink and tub drains; not clear to me whether this would be as effective as putting the peppermint down the vent (presumably it's much diluted this way?), but in any event we didn't detect any peppermint in the bedroom. Plumbers #3 and #4 we only spoke with on the phone, but (given what we described about the location of the smell) they were skeptical that it was plumbing.

So: how to proceed?

First of all, if it's not plumbing or an animal problem, is there anything else it could be, that we've been overlooking?

Otherwise, operating on the assumption that it's a plumbing issue, what to do next? Plumber #2 says they're able to do a smoke test, but I'm not sure I trust them, given that their idea of the peppermint test was to have the same person who handled the peppermint extract also smell around for the peppermint. Are there any other simpler possibilities that we ought to be chasing down before we start laying down bigger $$ for more serious diagnostic work?