If at all possible, use a plausible in-game reason for their PC's absence. The PC decided to stay at a local village. They were hired to deal with a one-person problem. They are guarding a supply cache. They are busy with their new lover/partner/sword/mysterious artifact. They are doing Wizard Business and must not be disturbed.
But if you can't think of a good reason, or nothing obvious presents itself, you may roll on the table below. Give the player a choice. There are some positive and some negative effects; they shouldn't be penalized (beyond the potential lack of XP) for missing a session.
I'd suggest writing your own tables for appropriate locations. Drunken Cossacks kidnap a PC and return them with a fine horse. Fire elementals steal them away for secret dances. A witch turns them into a new (they get better). Etc.
Some ideas stolen from this thread.
Where Did You Go?
|1||Tiny hands grab you from the darkness.||Tiny hands return you from the darkness.||-|
|2||Touch stone and get dragged in.||Vomited out from the stone.||-|
|3||Snagged by spider silk, vanish upwards.||Returned, cocooned in silk.||Start session at 1/2 HP.|
|4||Echoes lure you away from the party.||Wander back, exhausted and changed.||Roll on Effects table.|
|5||Fall down a deep narrow hole.||Fall from hole in ceiling.||Start session at 1/2 HP.|
|6||Dragged under in a sump.||Found unconscious near a stream.||Start session soaked in water.|
|7||Whisked away by a burst of shadow.||Returned as if no time has passed.||The Rapture's next target.|
|8||Vanish while the rest of the party sleeps.||Party member rescues you in a dream.||-|
|9||Transform into a tiny bone idol.||A drop of blood changes you back.||1d6 temporary Wisdom damage.|
|10||Trapped by a cave-in.||Loop around, harried by enemies.||Gain 1/2 XP from missed session.|
|11||Separated while traveling in a maze.||Stumble back, angry and confused.||Start session with 1 random item.|
|12||Cave madness. Run away screaming.||Return, apologetic and hollow-eyed.||Roll on Effects table.|
|13||Nearly invisible pit trap claims you.||Returned with a cryptic apology note.||Start session with 1 ration.|
|14||Bright glow, screaming, panic. You vanish.||Mysterious event repeats, you return.||Start session fully healed.|
|15||Take the lead on a climb, aren't at top.||At the top of a later climb, confused.||-|
|16||Slave-taking raid. You were slow to run.||Return with shackles, blood under nails.||Gain full XP from missed session.|
|17||Fall off a cliff in a tangle of rope.||Long, desperate climb back up.||Start session at Starving.|
|18||Touch a mysterious rune, vanish.||Appear near the party with a thunderclap.||Roll on Effects table.|
|19||Vanish into the darkness with a scream.||Doppelganger returns, with you in pursuit.||-|
|20||Ambush or fall. Party sees you die.||You return as if nothing happened.||Start session fully healed.|