Assuming that it is West guarding the base; simply change it to East if necessary.
1. Make a trigger covering just the base and its guards:
West, Present, Once
and give the trigger a name, in this example: base
2. Make a second trigger, centred on the base and large enough to cover the area where the Resistance might be spotted, this might be 1500 metres in diameter:
Resistance, Detected by West, Once
write in the On Activation line:
"_x setBehaviour {STEALTH}" forEach list base
then click on the 'Effects' box in the bottom lefthand corner. You're interested in sounding the alarm, so choose
Sound - Anonymous - Play Alarm (scroll near to the bottom of the list)