Definitely water inside.
Mine used to unlock trunk when I was switching lights on. Licence plate light's positive would feed the lock.

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e53 '00 4.4i Oxford Green II Freshly rebuilt tranny(2nd one) and transfer case/front shaft
e70 '08 3.0si Sapphire Black cinnamon leather 7 seats
e61 '06 530XI Sports Touring Silver - SOLD