We have a cottage in which we are getting very intermittent smells of urine (we believe). It shows up at odd times in one of our three bedrooms. Stays around for a few minutes and then after opening a window disappears. The bathrooms do not smell. I have checked the plumbing in the crawl space and the vents in the attic and can visibly see nothing. I tried lining the vent in the main bathroom to count out a leak in the wall . This did not help. All drains have "u" traps. Help!! What could it be??

If the plumbing isn't being used often the traps can dry out... I have also seen the vents freeze over (if you are in cold climates)... Or maybe you have critters.

Were there previous owners that may have had a potty training age kid or something?
What kind of flooring do you have?
When I moved into one of the houses I used to live in you could smell cat pee every now and then like you are saying. It was in the wood floor by the sliding glass door. I used some pet odor & stain remover spray. It works for other odors to.
Carpeting will hold in smells also.

We have a cottage in which we are getting very intermittent smells of urine (we believe). It shows up at odd times in one of our three bedrooms. Stays around for a few minutes and then after opening a window disappears. The bathrooms do not smell. I have checked the plumbing in the crawl space and the vents in the attic and can visibly see nothing. I tried lining the vent in the main bathroom to count out a leak in the wall . This did not help. All drains have "u" traps. Help!!!!!!!!!!. What could it be????
We had a sick cat and a urine problem. It was cleaned up
Well but when the humity is high and temp. is just right
We still get a wiff of it. If you have high humity this might
Be the problem.