As others have said, it’s the calipers leaking very small amounts of fluid into the pads. I’ve had this with several (5) sets of Shimano brakes.
I think if you use the bike regularly, it seems to be okay, but limited use seems to exacerbate the failure…I have no idea why.
You can recover your rotors with a combination of oven heating to 160c and then cleaning with a scouring pad and isopropanol. The pads can be recovered using hot water, scrubbing and Fairy liquid, but it won’t last, once the seals have gone…it’ll need constant fettling.
I have Hope on all the MTBs now. Still have Shimano Hydros on the road and cross bikes, but the front on the road bike is starting to howl, especially when wet.