One particular thing in Mercenaries is the "vehicular disguise". Basically, once you drive around in a vehicle, and no one is around, your faction will become the same as the vehicle's owner. For example, you get into one of the A.N. Hummvees near where you start the game -- provided no one is around, the disguise your merc will have will be that of an Allied Nations soldier.
When disguised, soldiers of that faction will not attack your vehicle (if they are hostile towards your merc). They will pretty much leave you alone so you can drive around and go anywhere in their presence until such a time that the disguise is blown. How? Any of the following actions will result in a dropped disguise:
Honking the vehicle's horn.
Killing a soldier in disguised vehicle.
Ramming another vehicle while disguised.
Attacking while in vehicle disguise.
Having any officer see your disguised vehicle.
Once the disguise is lost, the only way to regain the disguise is to exit the vehicle and re-enter it once there are no other people around (this includes occupied vehicles). The basic rule of disguise is make like a Larch (no. 3 ... the larch ... the larch) and not be seen.