Combine Door Lock

From Combine OverWiki, the English Half-Life Wikia

A Combine door lock is a small, metallic device that is placed on a door instead of a door handle or covering, and acts to keep the door locked in order to prevent unauthorized access. Locks are most commonly found in the residential apartments throughout City 17. This could possibly be because the Combine, in its oppressive nature, chooses when the citizens are allowed to enter or leave their apartments, and monitor everyone that walks in and out. It is evident that only Combine personnel may operate a lock, but Alyx can use her EMP device to override Combine locks in order to gain access to restricted areas, as she does in Nova Prospekt and beyond. Barney Calhoun can be seen activating only one in "Point Insertion" by merely standing in front of a door lock.

In Episode One, the door locks demonstrated the ability to self destruct, flashing red and emitting a beeping noise, then exploding, destroying the door they were attached to. This seems to have been used by Overwatch to forcefully break into buildings.