Alarm/Immobiliser draining battery problem

Hi - hope someone can shed some light!
I've a 1997 E230 estate (UK W210) - it would appear the alarm/immobiliser is causing the (new & checked) battery to drain if
left for a few hours - is this a known problem and if so what's the likely cause??
I've narrowed it down to this by manually locking the car and my problem has disappeared.
There's nothing more dangerous than a resourceful idiot
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.