I just tell them that I've been playing a while and should be heading out (which is true).
I was in a group once in Solusek B where a guy said he'd be AFK for a bit, then came back like 20 minutes later explaining that he'd taken a really big bong hit, passed out, and hit his head on a table. Was pretty nuts!
Had another guy in Crushbone the other day abruptly degroup, then send me a tell asking me to tell the group that he had to go and he's sorry. Why he couldn't just tell the group himself, I have no idea.
Honestly though, I don't see the need to get all worked up about it and lie. Just tell people that you should be going. Try to give some advance notice if you're an important class (tank, main healer); on the other hand, if you're playing, say, a wizard, and your group is already holding down the entire camp, so basically you're just dead weight, advance notice is less important
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