I am having this same problem on my 2015 focus ST. I've had the valves done on all 4 wheels because they were bleeding air from the valve stem, I thought this might fix it but alas no, it seems they are still leaking somewhere else.
I will add tho that I had a 64 plate focus that was just under 3 years old when I bought it and ford replaced all 4 alloy wheels under warranty. The mechanic told me they were corroded but didn't say where. He did say that ford alloys are made from cheap Chinese aluminium. Looks like I'll be replacing the wheels on my ST £££