I am sure that he sneaked in her parents' room last night. --> He _____________ in his parents' room last night.
must have sneaked
It is possible that Kane was responsible for this entire mess. --> Kane ___________ for this entire mess.
may/might have been
I am certain that I didn't leave the door unlocked before going out. --> I _________ the door unlocked before going out.
can't/couldn't have left