hydraulic disk brake too loose
As Nathan Knutson pointed out in a comment, squeezing the lever should move the pistons and pads inward. Do this with the wheel and rotor installed to avoid popping the pistons out of their holes (that would be messy). 

It seems likely that the pads will rub again unless you do something else to correct the problem.

Possible causes of pad-rubbing include:


The caliper is not aligned properly with respect to the rotor.
The rotor is bent.
The pistons are not able to fully retract after the brake is
applied. This can be caused by sticky pistons or worn-out piston seals.
There's too much fluid in the system. If the brakes worked well since the last bleed, this probably isn't the problem.