Urine "gas detector"
Yes, it's possible, given a large budget. The late 1960s-era manpack and helicopter mounted people sniffers tested in the US war on Vietnam could detect ammonia in human urine and other components of human odors using air sampling and IR spectrophotometer techniques. Not very effective militarily as human detectors, as they made a characteristic noise and were easily fooled by hanging buckets of excrement in the jungle and provided false positives from animal urine or odors from the soldiers using them. Detection distances in the hundreds of meters were claimed, but that may have been for something more like a latrine ditch for a large number of soldiers than a single man ("Charlie") moving discreetly. 

UV light can also be used, but it's not gas detection.