Police Suburban has no /locker

[Demonic]

Considering you're trying it on an Unmarked Police Suburban, no. It won't have a locker.
As it having a locker with weapons inside would be extremely inconvenient if you're trying to make your vehicle unmarked and undercover.
The normal Police Suburban however, does have a /locker command.

Spoiler :

Last I checked, atleast in belgium, we even have little mercedes cars with 4 men carrying MP5's and extra weapons in the back. And yes, It's a undercover car.
And normally undercover cars have blacked out windows... Normally it should have blacked out back windows and a /locker function... You can easily make a hidden comportantment in such a big vehicle.