Hi Martin,
There has been considerable conversation about this matter and I think it is scheduled for the next summit to create a rule,
I think the consensus is that if you are satisfied they are not returning the chips come off straight away and they would then receive the relevant position at this time including if it was a cash, and if there is any doubt as to their return the chips stay in play - how long for is a decision you must make as a house rule. I have a rule where if they have not returned after two levels their chips come off for certain games but that is sometimes not practical to the bigger tournaments.
If it was my game and he did not return before heads-up (i.e. his stack and two others remaining in the tourney I would bust him at the point he had left, and all other players would advance a pay scale higher, but that's just my way of doing it.)
There are no hard and fast rules ATM for the situation, it is down to your interpretation of the situation unless you have an existing house rule to cover it.
Regards
Stuart