Portal Alarm

A portal with this spell cast upon it shows no visible signs of magical tampering, although the spell can be discovered by detect magic. Nevertheless, if the portal is activated, the caster receives a mental signal alerting him to this fact. The signal can be transmitted to the caster as long as he is on one of the two planes connected by the portal.

The alarm is purely mental and can't be detected by anyone other than the caster. When activated, the portal shows no sign of special magical activity.