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.
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