Hand wave with a twist of justification.
"Rob's character is there, he's just, uh, scouting ahead/down with the flu/watching the rear/taking a really long dump/etc."
This also covers the sudden arrival of late players, the sudden departures of those who have to leave early, etc. My group is about 8 players- we have a lot of in-and-out going on, and my basic philosophy is, "The game goes on!"
As long as we have 3 pcs, and sometimes with less, we play.