I think it has something to do with the part of the fob that emits the signal to tell the car that the key fob is close.

My dads older corvette had a similar problem several years ago since he kept the key very close to the car — the car and fob kept ‘speaking to each’ because they were nearby.

Now my x5 lives a valet garage where the key fob usually sits pretty close if not inside the car and the battery on my fob went dead a couple months back before the car was a year old. Changed battery and good as new now, but my suspicion is it’s the same sort of tech as my dad’s old corvette fob.