Why would my apartment smell like fennel when we've never had fennel?

This odd smell tends to be more prominent at night and only appears in one specific area of our apartment. This area would be behind our couch which is against an outside wall with a baseboard heater. of course I've moved the couch and have not found anything behind or underneath it. It is not constant, and by that I mean there are some days we don't smell it at all and other days it is very pungent. The only thing I can associate with this smell is fennel. We've never had fennel in our apartment so it can't be that simple. Our apartment is on the ground level right above the furnace and boiler room. This mysterious smell has been in our apartment on and off for as long as we've been here...almost 5 years. What could this smell be from?