It seems like it does laser scans quite often. Is it so it can identify enemies, and, if so, why did it not do a scan on Jack before he said "Tech Four-Nine. Jack Harper."?
Drone #166 (the very first one Jack repairs, and the one I assume you are referring to) actually does a scan on Jack. Not with its lasers, but with its "eye".
The drones are equipped with both an "eye", which seems to be mainly used for navigation and close range scans, and lasers, which seems to be used for long range scans as well as to scan in multiple directions at once.
As for the second meeting with the drone (down in the New York Public Library,) the second scan, which is done with the lasers, is just there for some extra seconds of doubt to make the audience unsure about whether the drone will actually recognize him or not.
Drones scan objects and surroundings in order to find Scavengers. (Navigation probably is done by GPS and/or radar, because drones do not scan during flight.)
As for why Jack identifies himself, there could be multiple reasons. I think it is just human nature to prove innocence against an armed opponent and it could be a reflex of Jack on the just "awoken" drone. Furthermore, maybe Jack identifies himself just to speed up the scanning process in order to get the drone in normal operation mode quickly. But most important: it could also be protocol, because later on we learn that the drones are ordered to eliminate all humans.