I have a Warwick bass that is both active and passive. For the active mode it requires a battery, while for the passive mode it does not. While I have the instrument plugged in and on the passive mode, does it consume the battery the active mode uses?
Working in passive mode with the battery removed does not necessarily prove it does not use any battery when the battery is inserted. It may still leak current if the circuit is not completely broken by the switch.
If you want to know for sure, connect up a multimeter to the battery on the ammeter setting (Google how to measure current with a multimeter). It should read 0 in passive, and xxx mA when being used (actual consumption may be in your Bass manual)
By the way, also note that batteries internally lose their charge over time. And temperature extremes have a big effect too.
Or, you could just carry a spare battery...