I live in a home built in 2006. We are a clean family and have no pets. In ALL three of our bathrooms I am smelling an odor of "old urine". I do have a 3-year-old son so at first I thought he was simply missing the toilet in his bathroom, but days after both my husband and I noticed the same smell in the other two restrooms. I cleaned all with bleach thoroughly and still. . .the odor remains. Could this be something with our plumbing? Can we do a cleansing of the plumbing in the house?
Being a landlord I have tenants that just don't like to clean/scrub toilets weekly... try once or twice a year... however they do sometimes call up and say my toilet smells when I flush it. Every single time I have had this the solution has been the same with tenants... clean it with toilet bowl cleaner including under the RIM and clean it about 5 or 6 times as so much stuff is stuck everywhere... even change cleansers every other week.. but not in the same day (don't mix chemicals)...
Another possibility is the kid peed on the walls.. wash them.
Another possibility is ... are there warm air floor ducts near the toilet... maybe the kid pees in them for fun??? lift the covers and have a look... I did find one grill that was covered in pee spray... in a guest bathroom... once upon a time.
Another possibility there is a bad seal or no seal under the toilet... there is supposed to be a foam or wax ring under the toilet that keeps the air and water in the sewer system separate from the air and water in your house. I have had one instance in 20 years of a urine smell that happens every time you flush that cleansers did not solve... it turned out be a foam ring that was not wide enough to fill the gap between the floor and the bottom of the toilet and every time you flush - air from the sewer passes by after the water has passed by. Strangely there never were any water leaks that anyone ever noticed.
THe only way to check this is to remove the toilet by unscrewing the floor bolts, lifting and looking to see what is underneath... before you do this you might want to buy 3 regular wax rings and 3 extra thick wax rings... so you can choose which you want after you lift... if the rings are foam you might notice how one end might be squished and the other end appears full height... the full height is your clue that air might be getting through... Personally I don't care for the foam rings (hardware stores charge more for these than wax.. but it is harder to determine if you have a good fit.. with wax... it all just squishes out if it's too much and you get a good seal.
The last and least likely idea is that there is an issue with the vent stack for each toilet. Don't know the reputation or experience of your builder or the care with which city inspectors review builder work before signing off... but... you might want to check if there is a clear path from the stack inside the wall behind the toilet to the roof. Start with a quick look at each roof vent (the little chimney sticking out through the shingles) and see if you can see down it.. or is it plugged with a bird nest or some dead critter?
I had a shower that smelled like cat urine for a year or more no matter what I did. I sniffed around and found it was a mold that was growing unnoticed on the tile grout. The tile is brown and the mold blended right in. It appeared to be brown and somewhat slimy. I believe it originated on the connection of the soap dish to the wall which had a layer of slimy mold on a tiny unnoticeable portion. I scrubbed with mild cleaner, and vinegar. Nevertheless, after a week of showering it can come back. So I constantly clean the shower walls with a mild mildew reducing shower cleaner and brush. That keeps the mold at bay. I will try baking soda, vinegar and hot water soon, to see if I can get rid of it forever.
That particular smell is quite surely related to some kind of mold, probably the same that grows when you miss the toilet and don't clean it for a couple of days. Reading your question I thought about some leaking from the shower or the back of the sink, but if you have the same problem in your 3 bathrooms there must be a connection.
If you live in a very humid area it is very easy to struggle with this kind of trouble. Bleach is a good solution for most kinds of mold and you don't have to use it in a very concentrated proportion. Every wet spot in the bathroom must be taken care of and if you live in a place with high levels of air humidity, every spot is a potential wet spot. Open your windows as much as you can, even if it is raining outside, humans make houses more humid than outside.
I know it can be disgusting, but you can save tons of cleaning time if you get to know where exactly does the smell come from. If it is the plumbing, smells shouldn't come back, but if you live by the sea or some very windy place, it happens.