when a player skips a session

[mucat]

If the story is at a point where it would make sense for Steve-the-Unplayed to be off doing something else -- helping an ally, investigating a lead, meeting a contact, rsearching a question, or just not within shouting distance when the plot hook hits -- then that'll be what they're doing. But if they were in the middle of a scene where they can't simply vanish without scrambling the story logic, then they'll spend the session "over there".

The party is fighting orcs? Steve is over there, fighting some other orcs that were trying to flank you. Maybe he took along a convenient NPC or two. Their fight will finish when yours does, and if you win, so will they.

The party is negotiating a treaty with the Dwarf King? Steve is over there, haggling with the chief geologist about mineral rights. His agreement will be as triumphant or as disastrous as yours.

The party is cutting deals in a smoky waterfront tavern? Steve is over there, playing cards or chatting up a barmaid or something. When you're ready to leave, so is he.

Next session, Steve's player might (with the GM's agreement) fill in some interesting details about what was going on over there.