There is enough of water seeping through in case drain pipe is malfunctioning. If it's driven for too long, driver side carpet will also get wet. There is no other water source to wet the mat like this. Yes, coolant could be the one in very rare case because under-body cover facilitates it's reach everywhere. I noticed that in a Punto but then mechanic was intentional opened the nozzle of radiator to replace the coolant. Rahul, Switch on the AC & check where it is dripping by looking underbody. You can refer the manual if it has specified AC exhaust pipe in your car. Either the pipe is choked or it's not there at all.