In Alien, when they find the derelict spaceship, they're not really surprised. I've never been able to decide whether the humans in this univers have known other intelligent species or animalistic alien life forms. Seeing an alien spaceship for the first time at this time should have been a much bigger event, even for a bunch of underpaid truckers.
If they'd already encountered, say, dozens of intelligent species, and found the first traces of an extinct or existing new species, that should have excited them a lot more.
That's not really clear from Aliens either, although there are hints that they have had contact with lower life forms./Bug hunt/ But the alien shipwrecks should excite Ripley's interrogators and the others a lot more if they think it's his imagination.