I'd say you only have to make a Mobility roll if its Engaged with another PC (which is the Retreat action on page 59, basically). Otherwise I'd assume it just attempts to leave the area by the shortest route possible, by scuttling into a vent or hiding in the next zone, no roll needed as its a Xenomorph after all. Nothing is more sad than rolling Mobility for a Xeno, rolling no 6s and having it scrabble pathetically at the wall in an attempt to climb it. Though it can only move 1 zone away, because its Speed is now 1.
I had terrible luck with my first outing with a full strength Xenomorph. It utterly failed to Ambush somebody because I couldn't roll a 6 to save my life, then someone pipped it in the initiative roll and one-shotted it with a shotgun (despite rolling 20 dice for armour) then they got the "Blown apart" crit result. Maybe I should have fudged one 6 on the armour roll so it actually got an action, but I'm too honest, clearly.