Shimano R8070 Lever Leak


After reseting the pistons this week to put new pads in I notices a small amount of fluid run down the left hand brake lever blade, probably only a few ml but enough to cause concern.

Why might of this have happened, is there any kind of valve/seal in the lever that can open slightly to let excess fluid/air out?

Any help would be great, don't want to be riding round on compromised brakes.