Leaing it on the USB dongle shouldn't, and I stress the word shouldn't, discharge the battery in any signicficant manner. There is a diode, one way electrical gate, the is used for charging to prevent that from happening. Some very slight "reverse current" can flow, but it is so low as to be unimportant unless there is something wrong with that diode. Since the air switch or manual switch, depending upon what type of battery it is, is not being acivated there is no current flow.
Just my opinion, worth what you're paying for it, your results and mileage may vary, no warranty or guaranty is expressed or implied.