Big no to keys for me, much prefer pins. The keys will be a pain in the arse for both players and admins.
Currently, vehicles can still be stolen if the driver is killed whilst driving, is camped or you guess the PIN by using the thermal scanner.Vehicles can also easily be ransomed if left unattended by placing a satchel charge and demanding X number of poptabs. Vehicles can also be easily destroyed if left unattended.
Why not add another option. If a vehicle is left unattended for long enough (eg 30 minutes) it can be stolen if you "hack" it using a laptop, screwdriver, pliers and electrical cable. The "time to hack period" could be a set period with a random percentage of success (eg 5 minutes with a 50/50 chance) or possibly just a one off hack attempt which then locks out future hack attempts by anyone for a set period.