Have you check for animals living in your attic like squirrels or mice. If you have a crawl space check it as well. when rodents move they urine in the same area they do not go out side and it will create a strong ammonia smell in your house.
Hi, Welcome to JustAnswer.com, I am glad you are here! My name is Eric & I will endeavor to answer your question in a bit more detail looking at the issue from a different perspective.
There are several other things that can create an ammonia smell in your home. I am going to list everything I am aware of even if it has already been covered in the previous dialog and even if there is only a remote chance of it being the cause of your problem.
A. Previous owners or occupants whose pets urinated in the home. Over time, repeated saturation can soak through the flooring and dry but leave remnant and even dormant molecules. When it is humid, raining or when the wind blows from the right direction, the smell may be noticed.
B. You said you didn't notice it outside however, If there are dogs, cats or other critters urinating around your home, it may not be noticeable in the day time when the sun is baking on the areas however after sunset especially when raining or humid, the once dormant smell can easily come back to life.
C. Are you on a septic system? Do you have any open floor drains or drains that have not had water down them in a while? If you have any dry traps, you could be smelling septic gas. I know, I know... you know the difference between sewer gas and ammonia right? Well depending on the point it is in the life cycle of the septic tank, the elements and other mitigating factors, gas from a septic tank can sometimes be quite full of ammonia odor.
D. You said it is worse at night. At night you use lighting that you usually don't use in the day time. Check every light bulb in your home. Take them out and examine the bases for signs of burning and make sure they are re-screwed in tightly. Also check adapters, extension cords and receptacle expanders. Everything in your home that plugs in, check. The reason for this is that when an electrical plug or bulb is not connected solidly, or when the connection is not making good contact because of dirt or corrosion, it can sometime emit a smell that smells like ammonia. Especially the older Bakelite plastic used in converters and adapters.
E. This is the one I consider pretty far-fetched but here goes... Homes that were at one time in their history used as a methamphetamine lab tend to retain the smell of ammonia even for a long time afterwards.
F. Then there is the concept TheRon65 mentioned that of there being critters in the attic or crawl space. If it turns out to be that, please accept his answer.
That said... if you are inclined to try to deal with what you have decided is a urine smell, the best product to use is called Nature's Miracle and can be purchased at most pet stores.