If the outgoing Capt can be contacted, he will be asked to return to the a/c and sign the tech log.
If he can't return, a verbal debrief will be carried out and a local engineer will sign the tech log on the basis of the verbal debrief.
If the Capt is not contactable, the same process will be carried out with the FO.
If neither crew member is available, the local engineer will conduct a daily inspection prior to dispatch, and the incoming crew will conduct a full PDI and have the fuel contents verified by dipping the tanks.
It happens from time to time.