If you are not actually turning it fully off, but instead using sleep or hibernate, then it could be anything that is running in the background. Windows updates, malware, spyware, viruses, etc.
I would suggest turning it off (not sleep or hibernate) more often, this is actually good for it, versus being on all the time. I would also suggest you run some good scans for viruses, malware, and other things. And determine if there are any programs running that shouldn't be. Did you install anything new just before this happened? If so you may want to uninstall it and see if that resolves the problem.
Now if it is draining and the laptop is actually turned off, then either the device isn't powering itself down as it should be, or your battery needs replacing.