I once managed to flatten the auxiliary battery on my PHEV which also resulted in the wipers going (when they weren't switched on) and absolutely nothing else working at all - not even the hazards. I've had similar experiences with the new Ford Kuga PHEV - the 12v battery goes flat within a few weeks if not used. On the Ford the rear lights sometimes flash rapidly and the stereo speakers can make a thumping noise. The Ford battery (located under the boot floor, similar to Mitsubishi) can be re-charged from connectons in the engine bay, although if the battery is flat you can't open the passenger door which in turn makes it very difficult to open the bonnet latch!
In the case of the Ford Kuga, if the 12v battery is so flat that it won't charge from a battery charger, it can be done by placing a jump pack on to the battery connections (in the engine bay) and then the charger on to the jump pack. However, I've only done this on new Kugas with new 12v batteries that have gone flat. Might not work on an old, flat battery. Also I'll have to say 'Disclaimer!' don't try this at home kids, as this procedure might not be suitable for a Mitsubishi, and might blow it up or set it on fire, or something.