I found a workaround.

You'll need 2 triggers:

1. One main trigger (for alarm, backup etc)- your trigger you had issues with.
2. For enemy unit.

STEPS:

-Name your enemy unit "unit1".

-Set "detected by" trigger with timeout and type in:
CONDITION: this && alive unit1;
ON ACT: a = 1;

-In your main (alarm etc) trigger type in:
CONDITION: a == 1;

-There you go!

EDIT:
I found another and easier solution that can also be used with more enemies.

1. Name your player "player1" and your enemy "unit1".

2. Set "detected by" alarm trigger.

CONDITIONS: this && unit1 knowsabout player1 > 3 && alive unit1;

2.1 For more enemies:
CONDITIONS: this && ((unit1 knowsabout player1 > 3 && alive unit1) OR (unit2 knowsabout player1 > 3 && alive unit2) OR etc......);