^^^ Fairly common problem - check the hoses, pumps, connectors, everything on the reservoir that could leak. There are two outlets on the tank - the upper one is for the headlamp washers, the bottom one is for the windshield. If the tank only drained halfway down then the leak is in the upper one. Try filling the tank with water and looking at the connections, etc with a bright flashlight to see where it's dripping.
There are also some connectors in the hoses themselves, like the one inside the bumper for the headlamp washer line. Any of them could be leaking.