Hydraulic brake not retracting sufficiently off the disk? Sorry if this has been asked before (if it has, I haven't found it).

I have Shimano hydraulic disk brakes (type BR-M486).

The front one is beginning to make a slight noise even when I'm not using it:


A whispering noise like it's touching lightly
Happens even when it's cold i.e. when just starting out
Sometimes (but usually not) seems fixable temporarily by applying then releasing the brake


Apparently, when I squeeze the brake lever then the brakes close on the disk, and when I release the brake lever the brakes don't open again far enough to completely clear the disk.

When the noise happens it's slight but more-or-less continuous ... it doesn't sound as if only happens over part of the wheel's revolution.

Shining a torch/flashlight through it from behind, I can see daylight on one side of the disk but not the other ... so apparently, one of the pads doesn't retract off the disk. Looking at the rear brake I can see daylight on both sides.

I tried re-seating the wheel (to try to ensure it's sitting equally on both forks), which made no difference.

Do you (might I) expect a symptom like this if it's been too long since it was last serviced?

I don't understand the internals of hydraulic brakes: I can change a tire but haven't touched brakes. Is it something I could maybe fix, e.g. a screw to turn to adjust it, or should I take it to the bike mechanic?

Maybe this is the service manual for the brakes. Reading it, I see it says (towards the end),


  
  Remove the pad spacers, install the wheel, and then check there is no interference between the rotor and the caliper. If they are touching, adjust while referring to "Installation of the caliper".
  


Dare I loosen the "caliper fixing bolts", so that the caliper can move sideways? Or (instead of the rotor being too far to one side) is the problem that one of the pads doesn't retract properly, for some reason, so there's simply not enough total space between the pads for the disk?
You should indeed start by just loosening the caliper mounting bolts and trying to do a simple adjustment of the brake. Try to center the gap either by eye or by squeezing the lever while tightening the bolts back down, which sometimes works great and other times not at all. Tighten the bolts to about 8nm, which is firm but not super tight.

If that doesn't work, remove the wheel, remove the brake pads (careful never to touch the pad surface), clean around the pistons with alcohol and a rag, then stick a tire lever in and push the pistons firmly back in place. You've now reset the automatic centering of the brake. Put the wheel back in and re-adjust the brake. You have to work the lever a couple times until it firms up.

If none of that works, you probably have a sticky piston. If it's fixable, it's done by re-lubricating the piston seals with Shimano mineral oil.