I'd most likely go knock on the door of Brinks, hoping someone was there. I'd then accompany them back to the truck as they went to close it properly, and chat them up hoping for a bit of a guided look inside in the process.

If no one was at Brinks, I'd be inclined to see if I could easily lock the door and take a brief look inside in the process...nervous all the while thanks to the fantasy playing in my head that the cops would come screaming up the moment I touched the truck much as my logical brain would know how unlikely that was in such a mundane occasion and also expecting there to be no access to valuables from the driver's seat (if indeed there were any in the truck at all).

Whether or not I touched the truck or just drove away, I'd at least think about dropping them an email or calling the next day to let them know about the open door. Reading your post made me aware of just how strongly I think armored cars should be either locked or tended, and that the obvious, simple human error is a breach of security Brinks should know about. And I'm quite likely wrong about that...the actual security exposure is probably inconsequential.