Draining battery - immobiliser?

[Janpan]

Hi

We have a 1990 California 2.1 petrol engine (manual) - a LHD westy, which we bought last year.

We have always struggled with the drive battery draining but managed by disconnecting it when we weren't driving. We are obviously using it more and more as summer approaches so this is a pain in the neck. We have changed the battery cut out relay - no difference. Our mechanic (VW beetles and T25s) has checked the batteries (leisure and drive) and they are fine. They have also checked the wiring, which seems correct and as it should be. But it is still draining after about 48 hours.

The conclusion is that there must be something being left on that is draining the battery but we can find nothing; except that we had an immobiliser fitted last year (by the garage we bought it from) and it has a constantly flashing light and a relationship to the battery. Could it be this? Does anyone have any experience of unexplained draining of the drive battery?

I've only posted once before so forgive me if this is in the wrong place.