25 Things You Didn't Know About Alien
7. When Hiding In The Shuttle, The Alien Isn't Trying To Hunt Ripley
Many viewers see the alien's appearance in the escape shuttle in the final scene as its final move at successfully outsmarting Ripley and continuing to hunt its prey. However, a supplementary making-of book and some statements from Ridley Scott suggest otherwise.
It has been noted that the alien's colour changes from translucent grey to black over the course of the film, signifying a process of ageing and dying. Having successfully created new hosts (albeit ones that were incinerated by Ripley), it's likely that the alien was in fact looking for a quiet spot in which to die.
Backing this up is how it doesn't immediately attack Ripley - it only does so once provoked.