>Make sure to put a lid on it & don't be long getting back...
Why? You don't need to worry about infection as you're going to boil the whole thing later. And the flavor of the steeped grain isn't going to change just sitting (without the bag in of course) there for an hour or three.
I don't see why you can't finish the steep, bottle it up, and come back *days* later to start the brew.
(I should add that I'm a noob and don't know anything. But beer making *isn't* magic. Common sense says that steeping the grain creates a liquid medium in which to then brew some beer. Nothing magical should happen between the creation of the steeped liquid and the start of the boil. Now I could be wrong and something very important and chemical in nature could be required. But if so, I think the rules of common sense speculation require that I should assume *not* in the absence of any evidence that it *would*.)
(In fact, in a hypothetical universe, I don't see any reason, other than utter and complete lack of demand, a beer supply store couldn't sell pre-steeped brewing liquids and brewing liquid concentrates. I mean *yuck* and *huh?* but hypothetically, why not?)