Weapons had to be "green" no rounds in chamber/magazine or belt out of weapon when on the FOB. When you came into the FOB you cleared, when you left you locked/loaded "red". If he was entering or exiting a FOB, he would have had his weapon cleared.
Then again, that is how it was when I was there, things may have changed now. Although the lack of armor surrounding him in the pic, leads me to believe he was there prior to '06ish... but that is just a guess without seeing the rest of the truck.
As for bumper numbers, I was typing something up, and when looking for examples, I found this...
U.S. MILITARY VEHICLES: MARKINGS
Typically MP units number their trucks by platoon/team. As an example, the trucks in my company were numbered as follows
571-##
571 = 571st MP Co
6 = reserved for unit commanders
7 = reserved for 1SG, CSM, etc
10 = 1st PLT Leader
11-13 = 1st PLT 1st SQD
14-16 = 1st PLT 2nd SQD
17-19 = 1st PLT 3rd SQD
20-29 = 2nd PLT (same format)
30-39 = 3rd PLT (same format)
40-49 = 4th PLT (same format)
50-59 = Mechanics
80-99 = coms, medics, various other rolling stock
Trailers used the same bumper numbers as the primary mover, but added a T to the end. 571-34T (3rd PLT, 2nd SQD, trailer towed by truck #34).
Bumper numbers, vehicle specific markings (no step, TP##, etc) and some specific identifiers (Military Police, OC, etc) are allowed by Army regulation to be painted on a vehicle.
As for other markings, that was done by the soldiers themselves, sometimes by unit SOP. I had "Desert Rat Too" on the back of the plywood RV pictured above with "Insert Quain Here" as the gunner's name was Quain on the side. Often the driver's name and TC's name will be stenciled on the windshields, and sometimes the trucks name across the top of the windshield.