If your car has alloy wheels - most do these days - gradual surface corrosion is common at the tyre bead. It is also common for fitters of new tyres (or replacers of repaired ones) to clean off this corrosion to stop any leaks.

Just had this with Mrs BJ's Panda when we had 4 new Goodyear Vector All Season tyres fitted. After a while I took it back and they checked where it was leaking - I won't mention the National chain but basically the wrong type of weights had been applied on the inner edge of the alloy rim causing a slight weep. They cleaned up the rim applied a sealing substance and re balanced properly. All good now.