No, it wasn't that. You still have 'em in your team, if that happens, with the petrify symbol on the portrait. They just dropped out of the team - no portrait, no inventory, nothing - and stood there with a blue circle under 'em like they had nothing to do with me at all!
However, I conclude from your reply that it was a bug of some sort, so I shall continue and hope it doesn't happen again!
Thanks.
F.