Oil on the back of brake pads?
It's not good but it's a fairly common problem and there's nothing you can really do about it short of replacement. Shimano doesn't sell replacement caliper internal parts or rebuild kits. 

Oil there is capable of getting worked on to the pads or rotor, possibly causing sudden loss of braking power.

Pistons getting extended too far is something that should only happen by accident when the wheel is out. If it leaked some oil then and only then, that's not something to be alarmed about. Likewise, if it only leaks immediately following a bleed, or it had some leakage there when new but not again, that's annoying and not something it "should" do, but probably (and only probably) not an indication of bigger or imminent problems.

If it does it all the time in normal use, I'd recommend replacement.