Coolant Overflow Tank

It is not vary likely that a loose coolant cap on an unpressurized coolant overflow tank will cause the engine to overheat as long as the coolant level is where is should be. Yes, a radiator leak can cause coolant to collect at the bottom of the radiator. I would recommend you have the cooling system on your 1997 Honda Civic pressure tested to be sure where the coolant leak is coming from before replacing any parts.