My batteries usually work fine even after weeks inside the switched off camera. But I have also had experience of two batteries dropping voltage rapidly after I switch on the camera (not really related to being stored in the camera), and these turned out to be faulty batteries (not Canon original).
Since it's not really very easy to connect a multimeter to measure current drain, I would suggest charging the suspect battery, use the camera with that battery inside, and see how long it takes to discharge. That should tell you if it is a battery problem.
Richard
Since it's not really very easy to connect a multimeter to measure current drain, I would suggest charging the suspect battery, use the camera with that battery inside, and see how long it takes to discharge. That should tell you if it is a battery problem.
Richard