Gag, so sorry. We stayed in an extended-stay hotel for a month that had the same problem. We'd smell toast and other things the neighbors were cooking. We decided it was coming through the HVAC vents because whenever the heat would come on, the smell would be much worse. I'd wake up in the morning to toast and coffee smells. It was worst in our bathroom, so we kept that door closed. Fortunately that was short-term.

We also stayed in a second floor condo in Tucson that had horrible smells coming up from the unit below (we think). We tracked these down to behind our dishwasher. There must have been some holes in the walls or something. My DH thought it smelled like smoke (they were smokers), but to me it smelled like rotten food. We were only there a month so did not talk to them but did mention it to the property owners when we checked out so perhaps they could investigate and have any holes plugged up.

I have never tried it, but I did find this: http://www.pureayre.com/products-home.htm. Sounds like it has potential.

GOOD LUCK!