Does the liquid smell like antifreeze (coolant), it could be a leaking heater core

alternatively, there is a bung plug in that area. It enters the car below the battery tray. You can see it from inside the car with the footwell electronics exposed or you can access it when the battery is removed. Perhaps this plug has come lose.

You want to stop that leak ASAP. The passenger footwell house’s the TCU (transmission Control Unit) and amp for the stereo.