Thanks for the responses.
Like I said, my sense of smell seems to be not as acute as my wife's - I put it down to a long history of nasal congestion, allergies, etc.
Last night I could only smell something if I opened the cover and sniffed hard. I didn't think it really smelled bad, but there was an odor. Lots of times in the spring, when it is really wet, my gardens have an odor that strikes me as decomposing - as the winter detritus rots and new growth explodes. By itself, perhaps not a pleasant smell, but I always like it because of the future it signals. I think the odor I smelt last night might be similar.
I'll do it again tonite, and see if I can narrow it down.