There is nothing as heart stopping as when you get home and only one dog rushes out to give you a red carpet welcome. It verges onto panic when the missing dog is the mischievous one that gets himself into "uh oh" moments.
Luckily, we soon found out that it was because Rupert somehow got himself into a room that was shut off by a baby gate that closed on him and effectively locked him inside a room that he was supposed to be locked out of.
That's when we realised that he's learnt how to nose the baby gate open if it wasn't properly latched close. The only unfortunate side-effect was that when he barged in after nosing it open a crack, it tended to swing itself shut behind him, gathering enough momentum to swing into the latch and then he couldn't nose it open to let himself out. (That was also when we realised that when we weren't home, he's been patrolling round the house and sticking his nose into things.)
Yesterday, I came home to find this. He's learnt to use a stuffed toy as a door stopper so that the gate won't swing shut behind him. And he's so pleased about it too.