Common problem on all alloy wheels, firstly, over time, due to environmental conditions and/or damage, aluminium alloy corrodes becoming porous and expanding, this expansion forces the paint or lacquer on the alloy wheel to flake off leaving small gaps the air under pressure in the tyre to force it's way out (I think).
I have had this problem on every car I've had with alloys over about 8 year old, tyre fitters or garages can remove the tyre, scrape off the corrosion, put sealant on it and reassemble, I've found this to be hit & miss, sometimes it works, sometimes it doesnt.
I've been told refurbishing the alloys solves it, but it's an expensive fix. It's a frustrating problem they don't warn you about in the showroom when boasting that a car comes with alloys.
Only other fault I could think of is that all 4 wheels have leaky valves or slow punctures which is unlikely, only way to know is to put them in water and find where the air bubbles are escaping. Much easier in the old days when they just put an inner tube in, problem solved!