Giving the Boss the Bird

There is entirely too much nature around my office these days.  First was the bear, then a snake.  This morning, I walked into the office and heard what sounds like a couple of birds (or an adult bird & at least one chick) singing in the storeroom.  (Well, until I started typing this.  They've suddenly gone silent.)  The room has a door to the outside, so it should be easy enough to open it and let them fly out (unless it's a nestful that suddenly appeared overnight), but I'd really like to know how they got in there in the first place.