"If the Space Marines were defeated on a battlefield then it's probably not in an area that is amenable to Imperial interests. Plus, the hulks of vehicles would probably be too damaged by battle and time to be used without considerable repair. After all, Rogue Traders can get Rhinos through legitimate channels. Plus, there's always the chance that the vehicles you recover weren't used by Loyalist Space Marines ."
Of course if a vehicle bites you when you dig it up just cover it back up and run lol.