It's a fair question on just about all movies and games. For instance, when do characters or creatures go to the bathroom for a poop? When do they scramble around to make a meal? I don't make light of the question. I think if we get bogged down in these details, it takes away from the actual story that has to be delivered in a certain amount of time.
On a lighter note, it could be funny to see Ripley get a poo interrupted by a crew member screaming being murdered within earshot. "Who do I have to f&ck to leave me alone for a quiet poo?" Or the Xeno to scurry away from you and get caught sending an Instagram- "Stupid humans turning on each other because of me when they should unite. Napping in the vents before taunting them again. Maybe I will make some rattling noises or hiss a bit to keep them frosty."
For Isolation, I never wondered what the Xeno was doing- I was just glad it wasn't close to me.
