The Alien: From Egg To Adult By Smell
Egg:
Earthy, like sniffing a fungus growing on some moist dirt... until it opens.
Once opened, you would smell the most wretched fart. Not like a normal fart. The fart of an emaciated old lady wandering the halls of an old folk's home. Spoiled, boiled cabbage. The fart that tells you something's wrong with a person.
Facehugger:
This little guy still reeks of the egg for awhile, but it's own enzymes quickly clean it, giving way to a musky smell.
If dead & dry, it would smell much like the closed egg. If still alive & lubricated, more like patchouli & machine oil.
Chestburster:
Now of course the Chestburster is going to smell pungent & coppery when it's freshly birthed, being covered in the blood of whatever.
Shortly after though you would find that it smelled much farty like the inside of the egg. Maybe not as bad, probably like a freshly opened bag of BBQ potato chips. This is true of course until it begins shedding it's skin, at which point it would gradually give way to -
Adult:
The adult Xenomorph is constantly excreting lubricant from it's hide & drool from it's mouth which allows it to be a silent killer, a well oiled machine. The scent of the adult Xenomorph will vary slightly depending on it's environment & the amount and type of particulates it can absorb into it's body through the tubes on it's back.
Being biomechanical in nature I would imagine the adult Xenomorph has a very subtle scent if any, & that it would, if anything, smell faintly of machine oil or sex lubricant. If one were to encounter a cold, dry Alien that had been exposed to the vacuum of space for a brief period you might catch a wiff of motel room air conditioner just before it huffs it's pharyngeal jaw deep into your cranium.
When he's not covered in guts, the adult Xenomorph smells like the guy that changes your oil & is sleeping with his boss' wife. Start living right, Joe.
