Metro Workers Snagged in Sex Sting

I used to ride DC's Metro to and from work and school everyday. I knew and had semi-witnessed some overly sexual acts, largely at night when coming home from school, usually just a little overly aggressive petting. I have heard stories, but never actually witnessed, actual sex acts performed on the Metro, a la Risky Business. But, that was riders. Employees on the other hand, I don't think was what Metro had in mind for its system.

A Metro station manager and a Metro custodian were arrested on prostitution charges after an undercover transit police investigation found they arranged sexual trysts for money from inside the Dupont Circle Metro station.

At one point the employees used the Metro loudspeaker system to facilitate an illicit sexual arrangement, according to police who arrested the pair last week.

The story goes on from there.

But here is my question, how were the police tipped off that something like this was going on. Metro workers at the stations spend a fair amount of time working alone, so it seems odd that this would be occurring and that the police would get wind of it.