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Hi. Yes, it seems to take ages for the immobiliser to lose its sequence. Just changing the battery or doing some work with the battery disconnected won't do it.
Actually, the unlocking code is "burned in" and is never lost. But there is also a rolling sequence code that modifies the code transmitted by the key so that it is different every time it is used. This is to prevent the potential of theft by covertly recording and replaying a key operation. It's this rolling sequence code that can get out of sync if either the key or the immobiliser is powered down for a long time. It should be quite rare, and you should be able to find the resync procedure easily enough, although you have to dig for the exact procedure for your car.
Currently in ChrisG_UK's garage:
1991 Renault Espace RXE V6