Peugeot Boxer Battery

My AS Kingham on the Peugeot Boxer is just 12 month old and after returning from 10 weeks in France (4000+ miles) it was parked up for less than 3 weeks. Yesterday I was taking the van out for the day and was very surprised to find that I had a very flat vehicle battery. I am absolutely certain that lights, radio etc were off. I also have a solar panel and two leisure batteries that were still fully charged. Firstly I need to figure out why the vehicle battery was flat and I ask should the  EC 500 power control system switch solar power to the vehicle battey when it runs low. Any thoughts anyone please ?

The vehicle battery's fitted to Peugeot Boxers can be of two sizes. A 680 or 900 amp. 
I've just replaced the 680 on our late 2013 van with this one
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Biggest battery I could find and a top quality as well. Direct replacement for your van ...

The answer depends on whether you left the EC500 in system shutdown mode or not.  if you did, it does not charge the vehicle battery.

The owner's manual is here (as well as being included in the A/S owner's manual):

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The relevant section is 3.6 - Solar Charge Management.

Many thanks mojo....actioned as you listed .

Sylvester1954
You are not alone!
In the 5 years of my Peugeot based Broadway I have had 4 inexplicable instances of the vehicle battery being completely flat. After the first instance (the van was then 3 years old) I replaced the battery but since then I have had 3 more instances.
I have seen opinions that even if the radio has 'timed out' rather than been turned off, it still draws current in that state. I am of the opinion that there is a sticking relay somewhere but I would be interested to here from you if you find any definate cause.