Maybe the brake was engaged while the wheel was out and the pads just need to realign. Put the wheel in and loosen the brake mount screws, engage the brake and tighten them up again
This would probably make the pads closer to the rotor and lever feel sharper, or worse when one of the pistons pops out, but the brake fluid everywhere would be a bit of a give away.
Maybe the brake was engaged while the wheel was out and the pads just need to realign. Put the wheel in and loosen the brake mount screws, engage the brake and tighten them up again