Yes, Trexian's solution is exactly the same as Peanut's original suggestion, only not as simply implemented.
Now that I've seen your actual problem, I'd suggest something like this:
Activation: OPFOR PRESENT REPEATEDLY
Condition: this
Timing: TIMEOUT 60, 660, 360
OnActivation: player sideChat "Hostiles spotted in Birmingham"
This means they will always be spotted after being in the zone continually for 11 minutes (660 seconds), but any time less than that, between 1 and 11 minutes, they might be spotted or might not, but the longer they stay, the more likely it is. This makes it quite random when they will be spotted, doesn't make it predictable to them when they are spotted and encourages them to keep moving, lest they be guaranteed to be spotted.
Another alternative is to name the trigger "spotter1" and have a gamelogic with 50% probability of presense that has "deleteVehicle spotter1" in its init. This way, the logic is only created half the time and if it is, it removes the spotter trigger entirely. Again, it depends if this suits your mission or not.