There are many things you can try, and most work part of the time. I've yet to find something that works 100% though. Anyway, here some thing you can try.
Put a trigger covering the entire patrol, and the patrol area they walk. If the patrol members are side West, make the trigger activation West, and if the soldiers are side East make the trigger activation East. Name the tigger something like trig_patrolarea.
NOTE: Make sure you're actually naming it, instead of puting that in the TEXT area.
Now, in the init line of snipers type
doStop this; {this reveal _x} forEach list trig_patrolarea
That will make sure the snipers know about the patrol.
Also, create a "spotter" like a black op in a bush very near the patrol route, and group him with the snipers as their leader. Then add this to his init line:
this disableAI "MOVE"; this removeMagazines "HK"; this removeMagazines "Pipebomb"; this setUnitPos "DOWN"; this setCombatMode "RED"
When all of the patrol people are killed, just have the snipers first join grpNull and then deleteVehicle the "spotter".