Flat Aux Battery Investigation...........

Recently I found my aux batt flat as a tack, its on a dual battery isolator system and has worked perfectly for some time now.

I arrived at camp after a reasonable drive, setting up my panel on my solar reg, noticed the voltmeter at 8volts!!!!!!! shock horror!!

no dvm handy but starting vehicle didnt show to be increasing voltage,(using solar reg for voltage display) connecting panel did increase slowly.

anyway pulled all apart at home and all charging ok, no bad connections found, so i'm at a loss why it happened.

I was expecting to find either my battery (aux) or my isolator toast, but both ok.

Now in the last week, all i had done different from my usual habits was that my Engel actually sat in the vehicle on its slide and connected up, but NOT on.
(i'd been using it at weekend and as going out following weekend saw little point in removing from vehicle)

So my question is.......... will / does an Engel draw anything at all in switched off mode??

As if it does, even a miniscule amount, this would account for it over the week.

Cranking battery not affected by any of this.

Any pointers? explanations?

Rgds

Ron