There is an “urban legend” that some swimming pools have a dye in them that marks urine trails.
A chemical could be manufactured that would react with urine and leave a coloured trail. Unfortunately, it would be difficult to prevent it from reacting to other organic substances present in pool water.
A few pools capitalized on this urban legend by creating professional pool signs that warned that the pool was indeed being monitored with such a chemical.
Even the professionals do it, sometimes to put of other competitors.
http://www.dailymail.co.uk/tvshowbiz...ing-pools.html