[torresfe]

Try to screw down the treads on the rod. Align the piston with the correspondent notches on the rod.
Lubricate with brake fluid the outside of piston and push back in place.

Other possible way; will be to remove the 2 bolts that hold the electric motor in place.
align the piston with the correspondent notches on the rod. Turn the outside motor connection point
with a large male Torx, while simultaneously pushing on piston.

 

[torresfe]

...and recall - brake fluid is both poisonous and removes paint - so try not to get it on lots of surfaces...

Of all the brake pads and calipers I've replaced over several decades, I hadn't seen a piston pop out of its caliper, unless someone
was doing a caliper rebuild and intentionally disassembled it.

Curious...

The only way I can see this happening is by removing both rear calipers simultaneously. Replacing the pads on one side and reassemble.
Pressing on the pedal to make them sit back in place but forgetting the other side still removed or un-assemble. Just Speculating.