Take the pads out, squirt some water around the pistons, put something between the pistons to space them, and pump the lever. If the piston seals are leaking, you will see small bubbles around the seal. Another sign of leaking piston seals is always having small bubbles come into the reservoir when bleeding, even though the lever feels firm.
I’ve now had leaky piston seals on 3 different calipers I’ve used (M755, M775, M975).