Dump some water in all basement floor drains while you're at it. If it's only when you use the one powder room, you may be pushing gas out only when this room is used. If the basement traps are dry you could get the smell upstairs.

Another place to look for strainge intermittent smells is the sink trap. If it's used for makeup or if it collects hair, you'll need to take it apart and clean it once in a while. The first shot of water in there after it stands will push the smell right up in your face. The complaint for this one is often smelly water, when it's actually a dirty drain.