Mon 9th had steering rack boots replaced (not related to my issue) and went to fill up with gas and usual check tires air pressure, all was good with the tires.

6 days later got the gong noise and the red exclamation on the dash low tire pressure. Visited nearest gas station , and one tire was at 1.1 bar and not the usual 2.2 i had it 6 days ago, so filled up to 2.2 went to nearest tire place and they could not locate a leak/nail or bad valve. Next day in about 24 hours pressure was down to 2.0 ie 0.2 bar drop (same air pump used) , went to another garage they checked all good no leaks. Went to a 3rd no leaks and not the valve...
Annoying thing is none of them dunked the wheel in water, not sure if they were lazy or how feasible it is....all i can think of is a cracked rim or the lip of the tire again the rim leaks (but soap sud test showed nothing)could it be that it either leak when the tire is at speed or changes temp? I am stumped

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N