Central locking problem: unlocks all doors in cold weather

For the last two winters I've had a strange problem with my 1999 Ford Focus which occurs when the weather is cold (below about 12 degrees C).
If you unlock the drivers door with a single key press of the remote keyfob, about 1-2 minutes later, the rest of the doors unlock as well making a big clunking noise while I'm driving. It's not supposed to do this and I don't know why it seems to do this in cold weather.
Several other people have posted about this problem but usually nobody has replied or nobody has posted a cause or solution.
Does anybody know what might be causing this to happen?
I was thinking that perhaps some control box or fuse box has got water/condensation in it but I don't know where the fuse or circuits for the central locking system are to check.
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