Have the exact same problem. Our house is a 100-year-old home with a crawlspace. Plumbers, pest controls, checking of the crawlspace screening, all have been to no avail.
It comes usually every 1 to 3 months, but it may appear as close as every few days.
Your remarkable comparison to a ghost is interesting. That is what we call it. It is as if some invisible old person, incontinent, has come into a room and removed her depends. It is clearly distinct, different from the smell of the dead animal, Which should be called a putrid odor. It is also distinctly different from dog feces.
The "bloom" character of it is also exactly as you describe. It is simply there within a matter of a moment, and seems to disappear somewhat more slowly, completely gone within the half hour. With these simple venting of the room, opening windows and using the fans, the odor can be completely removed in about 20 minutes.
The only Hint of an answer is, after an extensive Internet search is that there is a chemical process involving water or humidity exposure to a mortar or plaster. The result is urea salts are produced. I am told that urea salts will be the actual source of the urine odor. The only problem is I cannot find a source of Water contact with either mortar on the perimeter foundation of this old house, and certainly not with the original horsehair plaster walls.
I have not checked Venting pipes from the sewage system through the ceiling. I should have all of the sewage traps filled with water. I have exhaust fans working in the crawlspace to keep it ventilated.
I hope my experience will help you find a solution. I very much hope to hear what it is. Thank you.
Cat Urine, consists of three, chemical parts the urea, which makes the urine sticky and allows the other two parts to create odors. Urochrome gives the urine its ugly yellow color that stains carpets, furniture the walls. The third part is made up of Uric Acid; the uric acid in the urine consists of salts and crystals that give off the extremely pungent odor that vaporizes and turns into an invisible gas. Spray’s and candles are just short term fixes until the next accident. A permanent solution, a product called AirReNu, which only requires one application, works night and day 24/7/365, removing all odors and improves indoor air quality.