I contacted a "bat lady" (from Wildcare Foundation) who explained that the bat could be sick or injured or just in deep torpor, a daily sleep state in which the heart rate and body temp go way down -- this happens at my office every day after lunch.
When she arrived to retrieve the bat, it was dusk and the bat was still very lethargic. She picked it up to hang it in her pop-up bat carrier and it got a little more animated. When I cat came by to investigate, the bat got even more lively so it looked like maybe the bat would make it. Carol told me the bat was a Colonial bat (evening bat) and could find its way back home...pretty cool!