I’m pretty sure the preamp still drains current when the bass is in passive mode, exactly like an effect pedal when it’s bypassed. usually there is some kind of build in switch inside the jack plug, which turns the electronics to “on” as long as something is plugged in.
at least it’s nice to have a passive mode ! 
Oh yes. If I didn’t have that, I might actually be making that drive for a battery. It’s been too many days since I played.
Oh hang on! I have two other electric basses and an acoustic. Duh!
don’t talk too loud about acoustic bass please, my GAS is sleeping … 
WAKEY WAKEY!!! 
I already bought two preamps in about 1 or 2 weeks, it’s not bad already for a “sleeping GAS” (I’m more an amp geek than an instrument geek !)
I guess you learned that lesson (as did I at some point) - ALWAYS unplug (even when taking short breaks - if nothing else then to get into the habit), and ALWAYS have at least one spare battery around (i.e., in your gigbag)!!
If you unplug, the battery should last several months with regular use!
spare batteries everywhere I must say this kind of thing is a part of why I don’t want to go back to active basses 
@PeteP - For all active or active/passive bass’s, always unplug at the bass when not in use. The amp plug completes the circuit for the active setup even though you may have it in passive mode.
Lesson learned. I do wonder why it’s a 3 way switch if it isn’t an off. Ah well. I had a little noodle in passive mode so all wasn’t lost. Thanks for the replies.
yeah but passive mode is cool, isn’t it ?
It’s not a three way switch. It’s a two way switch. Passive or active.
The preamp always drains the battery when the cable is plugged in. Never leave the cable plugged in. That’s the actual on/off, the passive/active is just a signal bypass.
The LED wasn’t on because the battery was completely dead 
My switch clicks over to each side and has a position in the middle. With it in the middle I get nothing at all, not even a little hiss or hum (even when the battery was working), and when I plug it into the B1on to tune up the middle position doesn’t even put a silent signal to it, it has to be in one of the other positions.
I understand that the socket is a contact switch too but I don’t get why I also have an off position - it’s obviously redundant and a little misleading. One thing it does work for is stopping me going from full volume passive straight into full volume active, right into my headphones. It acts to remind me to turn the volume down first.
Regardless of what that switch does, I’ll be unplugging from now on. A little irritating because I pick up the bass several times a day, often as I’m passing, and I already have issues with tripping over the cable because of the layout of my study. Hey ho!
maybe a kill switch (used on stage to cut the sound very quickly)
That makes sense. Not having played on stage it’s not something that would occur to me. So basically a “mute” option rather than “off”. It would make sense that it would be in the middle too, so that you didn’t go through the other mode to get to it. Mystery solved, thanks.
Might want to check your manual. The one on my 604 is only 2-way. The manual just calls it passive/active too.
So, three possibilities - the 504 is different than the 604, yours has an aftermarket switch, or the switch has a dead spot.
I’ll check my manual when I get home but pretty sure that switch is not supposed to be 3-way.
Edit: checked the manual, I’d misremembered, it doesn’t mention the switch - just the plastic overlay and (IIRC) a quick reference sheet I no longer have. But here’s the 504 control diagram on the web:
almost certain it’s only supposed to be 2-way.
Yup, agreed on the options. Unlikely to be different spec given all the other commonality (but not impossible). Aftermarket switch is possible but it’s been wired up that way and it’s not been replaced for its looks. A dead spot (due to a fault) isn’t the answer as it is a definite click into the middle and click to either side - it’s a 3 way switch, of that I’m certain.
@terb’s comment about a “mute” position would make sense, but you’d really expect that on the 604 too, wouldn’t you?
Looking at the Yamaha stock photo the switch is a flat toggle, and the labelled image shows the direction of movement being through a line roughly from jack socket to the bridge pick-up.
The photos of my guitar show it’s a round ended toggle and the line runs more back of bridge through to tone knobs. If the switch had ever been loosened than the direction it works could’ve been adjusted - deliberately even - but the 3 way spec is worrying. What else has happened to this bass?
Yamaha stock image;
My bass;
The manual that came with it is quite generic and doesn’t even show which knobs are which. In fact the diagram is of a BB model. It covers adjusting pick-up height, neck relief, tuners etc, but nothing model specific.
Cool, a bass mystery 
Definitely a different switch, and you can see where it was scuffed a little on removal.
That said, as long as it is working, I wouldn’t worry about it. There’s plenty of reasons for someone to replace parts on an instrument. Things break, wear and tear, personal preference, etc.
If you are concerned, take off the back cover and look inside at the electronics. It’s probably fine. You’re getting good tone.
That’s what it looks like until you look really close but it’s actually the little tab for securing it - like a washer but with angled lugs?
That’s a different matter: I need to record some samples and get you guys to listen, but I think there is something odd about the G string - it sounds like a harmony when played on its own (all other strings muted) and I’m convinced I can hear an oscillation that isn’t there with the other stings. It’s as in tune as ever. I’ll get those samples recorded and you can tell me I’m going mad/hearing things that aren’t there. 
I swear it’s that free jazz lesson messing with my head. Maybe I’m a Russian sleeper agent and the G string is almost activating me. 
The plot thickens! You playing with that switch just told your handlers you are ready to starting your pre-programmed assignment! Cool… 
