Beyond the urine trails from rodents, They will illuminate protein based stains from bodily fluids (urine, semen, blood, etc) and similar proteins from cooking (grease spatter, food stains, etc. on carpets, furniture or cabinets). They can also show where insects have been killed by spiders or splatted by humans.
I searched the house from top to bottom and it was surprisingly clean. I found only one rodent trail in the basement (probably from the mouse I trapped last year down there). Rugs are clean, furniture is good but the sofa shows where my daughter spilled soup last year, floors are clean too. Some kitchen spatter around the stove from cooking and minor spattering around the bathroom toilets from normal use between cleanings. The area outside and around my daughter's guinea pig cage is clean but you can see the urine inside the cage on the bedding. I found the spot in the garage where I stomped the giant spider a couple of months ago crossing the floor. I even checked inside the GTS...no vintage carpet stains from back seat 'fun'...LOL!
Interestingly, outside I was able to track the trails from our resident chipmunk right to his hole under the big rocks in the side yard.
Cool little tool and it's nice to know what's really around the house as far as that stuff goes.