The gate guardian heard the sound of the light exploding and fell into a panic.
Her first thought was that the stupid guard had been followed!
But then, she quickly threw that possibility out of the window.
If he had been followed, she would have noticed.
Additionally, no human enemy would destroy the only source of light in a room.
That would put both of them under the influence of the Nightmare, effectively destroying the moment of surprise of the ambusher, if there were one.
Both of them would be blind.
Unless the ambusher had a way to make light, but if that were the case, she would quickly notice since the light of the ambusher would also illuminate her, taking her out of the Nightmare's influence.
All of these thoughts happened in a single instance, and the gate guardian could do nothing but internally complain about the shoddy workmanship of the lamp.