Player Always Appears Online

In the campaign I am currently running, one of my players shows up as being in the game all the time.  Even though he hasn't been logged in for days, he still shows up (including after logging in and out again).  I trust him (and he wouldn't see any spoilers by watching me make maps, anyhow), but it is disconcerting to not be sure who is here and who isn't.

LM, what is the campaign name?  - Gauss

The campaign in question is "DCC: Choices".  Fortunately (or unfortunately), the problem seems to have gone away after yesterday's game, but the player's alleged presence persisted for nearly 2 weeks before that, even through logon/logoffs.  We are guessing that it might have been some sort of session bug, but I'll leave that to the experts.