Need to determine the source of the leak.
Is it possible it came from the overflow tube?
If the radiator was overfilled, or it got a little warm from the load and possibly grass stuck in the radiator fins, then some overflow is normal. The cap could also be getting bad. Look at the rubber, see if it is rotten or deformed.
If you suspect the radiator itself is leaking, fill it with coolant, look for leaks. If nothing, use a rag as a gasket, hold the exhaust from the shop vac over the filler neck to put some pressure on the system. Don't try this with shop air, too easy to blow out an aging radiator.
Also look at the water pump. Feel for loose bearings, look at the weep hole. A leaking pump can sling coolant in any and every direction.