Urine-indicator dye is a fictional substance thought to form some sort of colored cloud in a pool to indicate the presence of urine. It's commonly believed to exist in the United States, Canada, and Spain amongst other countries.
However, the substance can be found through pop culture over quite a lengthy time period. While clearly an urban myth a few companies capitalized on the general belief in the existence of such a urine detecting chemical by creating professional pool signs that warned that the pool was indeed being monitored with the chemical called "wee alert" or a similar catchy name. The signs were an amazing deterrent particularly for adults.