Possibly stupidly simple electrical issue on a recently purchased 2012 Jazz

[OilySock]

In my defence I've had plenty of flat batteries (on other cars) before - but nothing like this.
I'm baffled - does anyone know the answer?

I'll set the scene, the previous day I hadn't used the Jazz & it had sat on my drive, nose down on the slope, through a torrential rainstorm. Next morning i noticed the front LH 'postion' bulb was out (as per the manual it's 'position' but I'd call it a sidelight - capless 12v5w). I  couldn't find a spare in my stock but needed to set off for a 60 mile round trip on the M25, ending up back on my drive. The car performed without any faults, and before locking the car I made sure the exterior and interior lights were off.

About three hours later a neighbour let me know my brake lights were 'twinkling'. I checked and they were! They were flashing very rapidly and the 'you've left your lights on' audible warning device was belting away like a mad bongo player. Also, the central locking didn't work and the car wouldn't start. I disconnected the battery & my meter showed it was on 11.99v, which I thought was low but surely not low enough to cause this problem & so I was thinking in terms of a short circuit. I put in a new sidelight bulb & reconnected the battery. The horn started beeping, but locking and unlocking using the key fob stopped that, and the brake lights and bongo player had returned to normal. However, now every light on the dash was flashing on and off so I didn't even attempt to put the key in to start the car. I put the battery on a trickle charge overnight & went indoors to try to understand circuit diagrams.

This morning the battery was showing 13.04v, I popped it back in and the car is completely back to normal.
So, were the problems caused by the battery or by a short due to: water in a connector, bulb filament going rogue, dodgy wiring that I might have disturbed changing the bulb, or what?

I suppose time will tell, so I'll monitor the battery state & will update in a week. If there's no update look for a twinkling red Jazz on the hard shoulder of the M25...

Cheers

[Keekster71]

I would hazard a guess that water has got into the wiring somewhere and shorted it out.  Obvious places would be the fusebox or wiring in the engine compartment particularly if there are any relays.  I'm wondering that if the car is sitting at an angle in heavy rain, might result in the water getting trapped rather than draining away.  The rain is supposed to get caught in a black metal tray that sits under the scuttle, from which it drains.  I'd check to make sure to drains aren't blocked. 

[Lord Voltermore]

My guess is also short circuit due to water.  Car bodies are normally designed  so that when the car is parked  on level ground  any relatively flat panels still  have a slight downwards slope so water naturally drains away .To the ground or into strategically placed  drain holes.

My driveway is quite steep. Any car parked nose down it can reverse the level to the extent that areas that should drain away now form pools.   Most noticeably  along the  bottom of the tailgate  shut.    But maybe also  in less visible places such as light fittings, or the scuttle area .(below the windscreen where  wipers park etc)  I've not had electrical failures or noticeable leaks but suspect it might increase interior dampness.
  You can also get pooling on level ground if drain holes get blocked by dead leaves etc. I've even had insects and wasps setting up home  in light fittings, which could affect  drainage.      Its worth checking as many likely nooks and crannies  as you can for signs of suspicious dampness  or pooling. A while after rain stops but before  the sun dries it out too much.  

11.99 volts is not dreadfully flat and IMO would not  itself have caused the  electrical gremlins. But might be the result of parasitic drain due to the short circuit.   I think the fact you disconnected  the battery to recharge it  had a serendipitous effect of also  resetting the cars ecu computer.