It's not likely that you left a light on - the battery saver should have kicked in to prevent that (leave your overhead light on and it will automatically shut off after 10 or 30 minutes).
Modern cars can drain a battery if left for more than a few days. The PCM is still running, managing the anti-theft system and awaiting your keyless entry command.
I'd have it checked just to be safe - it is possible the battery is bad or there is something else draining it too fast, but don't be surprised if it comes back as normal.