Daniel over at System Matters recently posted a list of 20 reasons why one of the PCs is missing. It's in German, but he generously agreed to let me post a translation here. Some are more flexible than others, some are more specific than others, but I think they're a pretty good starting point for handling this kind of situation if you don't just want to handwave it. You can either use them as a random explanation table, or just improvise around one of the suggestions below. Any credit is due to Daniel, any poor phrasing is undoubtedly my hurried translation.
Who doesn’t know the feeling: a game session with your mates and someone’s missing. Chiz chiz… what to do? Call it off? Pull out a board game? Play a one-shot? But what if you really want to charge ahead with your campaign, because everyone’s having a blast with it? How can I explain the PC’s absence? No problem, just roll on the table below for a quick and dirty explanation of why the character isn’t around today.
1. Note: Not guaranteed!
2. Note: You can find a discussion of this topic in Episode 6 of the podcast.
Also in German!
01 – The character has been kidnapped, and needs rescuing
02 – The character is hanging back as a rearguard, and if possible, has had a little adventure of their own that can be played through at the start of the next session.
03 – A personal commitment (audience with the king, a visit from the in-laws, an inheritance, accounting deadlines) has prevented them from participating.
04 – The classic: badly injured or knocked out, and left with no memory of what’s happened.
05 – The character wouldn’t be dragged into THIS adventure for ten thousand ducats… only to reappear during the next session, for after all, he can’t leave his friends in a scrape.
06 – A magical curse like Flesh to Stone or Imprisonment makes sure that the character’s out of the game for a spell.
07 – Just can’t shake this damned cough… and then the rheumatism too… it’s best that you go on without me for now… I’m sure to be back in form soon!
08 – The character knows someone in this adventure that he’d really rather not run into again: an old flame, an old enemy he couldn’t handle, an old friend he ended up hurting, etc.
09 – The character wanted to make the meeting, but something got in the way: the plane wouldn’t start, his horse threw a shoe, the caravan made slower progress than expected, the weather’s unfavourable…
10 – The other characters’ message didn’t reach its destination: the host of the “Prancing Pony” didn’t send on the letter, a messenger was shot, the letter went to the wrong address, etc.
11 – The character got the job of rustling up some important stuff. The potions of fire protection for the dragon-slaying, the excavation papers for the local authorities, the military support for a siege, etc.
12 – The character was badly wounded before the adventure, and during the adventure something can be found to heal him, such as the Grail, a hermit with magical powers, Athelas, etc.
13 – Wine, women and song have left their mark. The character has simply slept too late to make the meeting punctually. This also works if one forgets to dust the mantelpiece after an evening with a bunch of greedy dwarves (see 10)
14 – Professional obligations! The Professor has lectures booked, the Lord is required to sit in the Upper House, the soldier is needed in battle, the farmer has a field to prepare, etc.
15 – The character is a persona non grata in this district. He’s on a wanted list, hasn’t paid his debts or is considered a troublemaker.
16 – The character doesn’t want to be dragged into this matter. His situation or status forbids it. A priest in the Castle of Maidens! The heir to the throne in the City of Thieves! A Cambridge professor in the University of Oxford!*
17 – The character is scared of getting involved in the adventure, because of a prophecy predicting that it will be the death of him.
18 – A strange power has taken control of the character! A Stygian wizard with mind-controlling charms, an Insect from Shaggai, an elven sorcerer or the power of a god leads the character doing extraordinary things. Use with caution!
19 – A messenger appears and hands the character a message; he rushes off at once. Perfect for starting a new adventure next time, and seeing what sort of a mess the character’s got himself into.
20 – The character slips ahead to reconnoitre. He should have been back ages ago… what’s happened to him? Has he been discovered? Has he found a secret passage whose door closed behind him? Is he in the treasury, stuffing his pockets? Has he found a friend in an unexpected place? Or is he *gulp* dead?
*Note: this is, indeed, in the original German version.