What is the hose located directly below the heater core hoses on the firewall of a 2000 ford focus ZX3 and where does it connect too?

I have a 2000 ford focus ZX3 which recently blew a freeze plug. I
replaced the plug but noticed a pipe located directly below the heater
core hoses on the firewall that has water pouring out of it after I fill the
coolant reservoir. It looks like it should have a hose attached to it but I
cant find where it would lead to or what its called. I got the idea to plug
it which seemed to work at first but after driving about 15 min it started
to overheat and had depleted the gallon of water i filled into the
reservoir. Filled it up again and still not noticing any leaks gave it
another shot, overheated again so I turned on the heater and water
poured into the driver and passenger side floorboard. Any help is greatly
appreciated. ~Z