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Must have a small puncture. Have a look for offending object in tyre. I once had one that only went down 2 psi a week and it turned out to be a pin with the head ground off and I needed to put the wheel in the bath to find the bubbles. Just hope it is in the central area of tread so repair is possible. Usually £10-15 at local tyre service.
It could be the actual alloy - had the same problem, took it to tire place - no leak! went home, week after it was down again, took it back - no ******* leak!! Turned out the tire didn't grip the alloy as it should, he said it's quite a common problem. ended up getting a new tire due to a blow out - all sorted now!