She has been great with the kittens so far, but this morning she shot out of the door at breakfast time (as she has been doing for the past few days) but we had to go out for the entire morning half an hour later, so had to shut the door.
I left a window open in the utility room (where the kittens are) for her but I've never seen her come in or out of this window - it's quite high and she can't see it's open from ground level as it's set back into the wall - and there is still no sign of her.
DH says she did the same on Monday and was gone for "a few hours" but it's been 5 hours now and we're a bit concerned.