Ah yes your theory sounds plausible - I like it! Though I have to say, if you're going to commit crimes in an active warzone at least have the decency to register your getaway vehicle properly.
Perhaps they're part of some underground Gibraltar car club that refuses to conform to yellow plate standards. "Yellow plates are for conformists" is their slogan maybe.
Aye, I can well imagine it now. A band of bold miscreants defying the sacred hue of yellow! The townsfolk must whisper in scandal whilst these scoundrels polish their white plates in proud rebellion. Truly, such villainy, and yet… I cannot help but admire the commitment.
