Just a note that the lock you found may have not been abandoned.

Many bike posts in my university's undercover bike shed often had several locks without a bike in sight. It turned out many people leaving bike locks on posts they often use (it doesn't take up much space or prevent someone from using the post).

It saves those cyclists the hassle of carrying the the lock with them (only the key), which is particularly an issue with those very heavy bike U-locks and D-locks.