RE:Make car stop and get out if encounter enemy
doStop [vehiclename];unassignVehicle [unit] (or {unassignVehicle _x} foreach units group [unitname])
You could change behaviour, set combat mode, etc...if necessary as well.
How do you want to trigger it? If you know the group that will be encountered, you could setup a trigger that fires the above code with the condition:
[patrolname] knowsAbout [badguyname] > 0
Or you could use a large "West Detected by East" trigger, or do it more elaborately by a frequently polling script that checks if the patrol group knows about numerous bad guys, etc, or do a real simple:
[patrolname] distance [badguyname] < [somevalue in meters]
which would make them stop and get out if they get close the bad guys.
RE: Discovering bodies - there is an outstanding script in the Script Library that deals with just that topic - I *believe* its by Snyper, but I may be mistaken. Try searching on "hide body" or something along those lines.