There is another way to get a seemingly random trigger.
Set a SMALL zone within the patrol area. THAT triggers it. If the player never patrols there, tough. (that obviously makes more sense for a campaign).
Alternately, you could be explicit in the patrol orders to patrol for X days at the point marked, then proceed to some other point and the hidden zone is there or en-route (in which case it could be bigger).
tater
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