[Sebastian doesn't care much about putting up warning signs - if anyone is brainless enough to walk on a unsteady bridge, that's their own problem. Besides, plenty of people are taking care of that already.
Instead, if Lucifer should eventually make it down to the underground cafeteria he'll find Sebastian there, busy chopping herbs and root vegetables for freezing and drying. On the off chance the world really is collapsing, and all its technology with it, they better have a stock ready.
He looks up only briefly when Lucifer enters.]
Welcome back, sir. You look like a person hunted down by all the devils in Hell.