In my experience it's uncommon for a Volvo caliper to suddenly start leaking. In our Nova Scotia, Canada climate, they usually just seize up. Do you know how much is left on the rear pads? It's possible, though again unlikely, that the piston in the caliper has travelled past the seal - you'd almost certainly have noticed the screaming of metal on metal, though, because this only happens if the pad is worn down to nothing - or missing entirely. Check the bleeder screw, make sure it's not the source of the fluid. If the rigid brake line has been changed from the original Volvo alloy to regular steel line, inspect it for failure caused by corrosion. Bottom line: put the car on a jackstand and pull that wheel off, get an assistant to apply the brake & find out where it's leaking.
-Chris