You're a terrible person unless you hint at some point that there is a genius supersoldier leader who can see them without them seeing him.
In cases like you've described there's no reason to not outright tell them to f$#@ing clear it the first time. You're going to waste their time, kill them, and then have to redo all the character creation.
That's just like, your opinion man. I absolutely hate it when GMs play my turn for me. Deciding to retreat or push is a calculated decision and a good chunk of the thinking and fun.
I'm not hinting at anything, beyond the fact that someone organized the dungeon. If they fail to ask any of the creatures or case the place before the end and investigate beyond just kicking down doors and killing everything, then they must not be curious what lurks at the end.
Besides that, I think it is funny.
Ahh there we go that explains it all now.