The grenade is fortunately not (yet) under my bed as the blog liked so much by all our big friends explains. Forget which one (it was in Czeslaw Miloscz's novel, which caused a whale to pick it up, or so he deftly found a good reason).
The cold air would already be streaming in - that's how it got under the bed, and furthermore, how the boy realised something might have been tossed in the first place. He woke up wondering about that draught of cold air. .... now I come to think of it, there might ! even be a grenade under my bed !- referring to no.24 in self same blog - and would I ever know, if it's one that failed to go off? In the summer with all the windows open anyway... I think my concern was more: what are the chances of stirring it to go off as yet? Will a creaking floorboard suffice (when running away) let alone handling it?
Hope you haven't had to find out in a practical class in the military, like we had to discover what phosphorous or magnesium does in a chemistry class.