That's waaaaaaaaaaay too complicated...two triggers?
Corbo's Way Easier Way>>
Conditions:
+Whatever conditions you want
Actions:
+Set doodad state(disabled) for all (beacon) at (Location)
+create 1 (detector unit [it can be observer, science vessel, even turret])
+wait 10 milis (or if you wanna risk it no waits at all!)
+kill all (detector unit)
+Comment (corbo pwns) [Optional]
Note: that the location must be covering the beacon you want to disable, and that the trigger might not work if you do not place that comment

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But then you are given vision of that area.