The passports were found dumped at the post box on three different dates this month, a senior postal department official said.
The official said while some of the passports were current, some were expired ones.
There were also some foreign passports among them.
The area is located about two km from the airport and police were probing how the passports found their way into the post box.
Meanwhile, police said various angles, including whether the holders had lost their passports at the airport were being probed or whether it was part of some organised racket.