You are not going to drain a battery through a switch that is switched-off.
However, if your vehicle has been “customed” wired, then there could certainly be circuits that have been wired around the switch and therefore will always have power with the key off, and could drain your battery.
An easy test to see if you have any battery drain is to use a multimeter that has DC current reading ability.
With engine off, key off, and all accessories off (example lights, fans, radio, etc) carefully disconnect one of the battery cables and insert your multimeter configured to read DC current. If the meter registers current, you have something draining your battery. Do not attempt to start the machine, nor even key on, with your multimeter in circuit as it will likely damage your meter or best case blow its fuse.