Could it be HBHF sensors by players are not detecting admins?
I just had that happening.

when admin is vanish still detected?
I found this hook

object OnSensorDetect(HBHFSensor sensor, BasePlayer player)

I guess it was because the player didn't build the sensors correct.
He built 4 of them and at last only one of them reacted on me.
Weird was that the ones that didn't react on me did react on him.
That's why I thought that they don't react on admins at all.

HBHFs do have settings to toggle whether authorized or unauthorized players are detected.  Could have chosen wrong when they set it up.