do the aliens smell bad?

I imagine they have a somewhat metallic smell crossed with an unpleasant but not so much pungent organic smell, something unfamiliar and unsettling in nature.
If they’ve eaten or killed something human they’re going to have that in their breath no doubt.
It’s a good question.

I think they might smell like oil.

I don't think they have much of an odor. It would give away that they are near.