[itsahoot]

stopping at each mail delivery point and I believe it beeped to alert a person to come and pick up the mail

There was a scenario in the series The Americans where a mail robot was in use at the FBI the sleeper spies compromised that mail system almost immediately.

[Thank You Rush]

I wanted to watch that show when it first came on but I got so tired of everything being filmed in the dark, I gave up and never went back. It sure SOUNDED interesting.