I have gone many different directions when explaining character absences in RPGs.
Inspired by this post at the Temple of Demogorgon (all bow down to its name)
We had the classic "they were in the bag of holding" routine, as I had long since ruled that if you stuck a tube or straw out of the entrance you could draw oxygen from outside of the pocket dimension. My old 3e campaign took this to another level as two ninjas in that campaign would tie a twine around the bag of holding and attach it to an arrow they would launch from inside the mostly drawn bag, shooting themselves around like little magical fucks. And I let them do it in the name of cool.
We had the "we don't give a fuck this if fucking Mutant Future and shit is all kinds of crazy anyway" approach. (sorry totally unrelated link)
In my 4e game a player has been unable to attend for several months but luckily his goblin tinkerer had made an enclosed gunner's turret inside the chest cavity of Hammer the war-forged Barbarian and at the end of the last session he played he was in a trance, his consciousness exploring the dark crystal heart that Hammer's essence had been fused with. So that dude can come back any time and literally just step out of another player character.
But in this campaign I am talking about starting up, I want players to drop in and out because of espionage.
Secret missions. Everybody is working for one government or the other (or both) and no one is surprised if someone gets yanked from one mission to do something else for a bit. Nor is it surprising when spies drop back in unexpectedly. There is a home base to get back to before you run out of oxygen.
Spies in a backpack, y'all.