How can I identify a urine type odor I can't find?

Question:After purchasing a condo we have a pervasive animal urine tye odor that drifts throughout our house. We have had multiple inspectors in to no avail. No Urine traces anywhere, we've used blacklight, infared, moisture readers, cut into our floor to look around and a number of other things. We're at a loss as to what it is.

I'm am now doubtful that it is in fact urine but an odor very similar.

-Our condo is from 1912, subfloor is the flooring & we have plaster walls.
-The odor builds during the course of the day, fading down in the mouring hours.
-There is no smell outside, above us, below us or even next door... we've checked!
-We don't have any crawlspaces, nor attics.
-We've brought in gas, animal control, mold and air quality people. No one has a clue but say it is a Urine type smell.
-Previous owner did electrical & plaster work

I'm looking to find the odor to solve it, not just cover it up.

Suggestion are welcome. Thanks

Answers:
A couple of other things to check. I'm a little suspect of the plaster work performed by the past owner. Do you know why he did it? I have rentals and there were pervasive urine odors as a result of two types of rodents, one with wings, one without. Heat seemed to "activate" the odor.

We literally tore a unit apart to discover that mice had at one time made a nest in the walls and saturated the insulation with their urine. By that time, the mice were gone, but that urine odor remained and we had to remove all of the insulation, sanitize the wood framing, remove the drywall (where you could see their urine stains) and replace everything. In the other unit, it was pigeons roosting on the roof where the flashing had become unattached and their droppings, etc. came in through an AC hose opening. Same thing only we had to rip out the ceiling and walls due to the location. These were both two story units and the rodents (I think) gained access through the roof. We recaulked, resealed and replaced animal guards and anything we suspected gave them access and put up barriers to pigeons and other birds roosting. So, check the roof to see whether there could or was at one time, be animal access, nests, etc. In our case, it was in the walls and insulation which was not detected by black light, animal control, air quality, even the electrician or moisture readings because it was dry by the time we got to it.

Good luck. I absolutely know how frustrating this can be.