Perhaps the coolant was overfilled. It will expand because of heat or pressure and the excess makes it way to the coolant refill tank. Make sure your coolant doesn't have oil in it. In a worst case scenario you could have a faulty head gasket or engine damage that allows engine oil to seep into the coolant. But if you don't see oil in the coolant, then just drain some off.
You need to explain more on where exactly the coolant is leaking. There are several possible reasons why your coolant is leaking. If it's from your main hoses that lead to and from the engine or radiator, you need to check the condition the hoses are in. If they're in a decent condition, it's not that. If it's leaking around where the hoses are connecting, it's probably not tight enough, or there is possibly a clog in your coolant line. Also get your radiator checked. If it's leaking into the passenger compartment, it's most likely the heater core.
If it is just water and not coolant, then suspect a leaking windshield, or possibly a A/C condenser drain line stopped up. If it is coolant, then the heater core is leaking and needs replacing.
could be leaking from a head gasket and trickling down or your heater core is leaking and driping out of the condensation drain from inside the cab. most likely its your heater core. check the fire wall above the starter and look for that drain line. if it is dripping and is colored (green or orange) its coolant and coming from heater core. that's a bear to replace. good luck.
your rad has a hole in it or u have a brokin rad line
there might be a leak some where in a coolant line. or maybe you have a leak in your radiator.
Replace the fitting.
Plugged A/C condenser drain line if it is plain water. If it is coolant, then the heater core is leaking. Another possibility is a leaking windshield.
I'm going to say that the vehicle is low on engine coolant.
heater core has a leak replace heater core
You may not have enough coolant. If you've recently drained and replaced your coolant, replaced all the coolant after replacing a leaking line, etc., you may have an air pocket in the system, which will need to be 'burped' out.
It could be transmission fluid. Many transmissions are cooled by running a line through the radiator. Check for small tubes going into the radiator, if you only have the two large engine coolant lines, you don't have a transmission cooling line in the radiator.
If it is just plain water, then you have either a leaking windshield or more than likely a stopped up A/C drain line. Look under the car passenger side and find the drain line and unclog it. If, on the other hand you have coolant leaking, then you have a leaking heater core, which must be replaced. Taste and smell the liquid. If it smells and taste sweet it is coolant and not just water.