[Clueless]

Help! We have a strong urine smell in our powder room that is worse when it is cold outside (below 40 degrees). We've had a plumber come out and 1st he told us that we needed caulking around the base of the toilet. That didn't help, so he came back and said that when the toilet overflows the dirty water gets in a little compartment on the toilet and sits there. We bought this house 6 months ago and the toilet has never overflowed since we've lived here. I think the plumber is blowing us off because he works for the Home Shield Warranty co. we have and they do not want to fix the problem. I've read about sewer gases, but I'm not sure if this is the root of the problem. How can I find the source?