I used to have this when I first started using the car after a winter layup, it lasted about 4 weeks and then went away. I had the exact same experience on my Mini when it was parked up for 6 weeks, then it went away.
My theory is that when the car is parked up for a long time the disc surface gets surface corrosion from rain exposure but the area where the pad is does not. when the brakes are then used the surface corrosion is rubbed away faster than the uncorroded section which leaves high spots. Over time, it all levels itself out.
You should be able to establish if it is discs by jacking up and spinning the wheels, you will hear/feel any high spots as the wheel spins.