Volter Manor - Position you shouldn't be albe to reach

I played a game of KF2 on Valtor Manor and it occurred that a player was standing in a spot in which he could not be attacked by any specimen except for husks and sirens.

Spoiler!

I dont know how he was able to get there. I only saw it when I died so he was already there.

Spoiler!

Volter Manor - Position you shouldn't be albe to reach