First off, for sure it's not trapped air - these systems are self-purging (I just went through 2 fluid changes, so know this to be true). And I doubt it's your radiator - symptoms aren't consistent with a blown radiator, and for the record these radiators last as long as the coolant is regularly changed (mine's

Second, if you've got a wet firewall, check the 2 hoses that pass from the back of the engine through the firewall and back to the heater core - they get baked by the engine and are known to fail (I bet against a failed heater core, plus I doubt a blown heater core would make the firewall wet, more likely it'd soak your interior carpeting!)

Third, if there's fluid on your garage floor, you've filled your overflow tank too - I'd empty it since it's supposed to remain empty...why carry around an extra 1/2 gallon of old fluid if you don't need to? To empty it, jack up the pass. side front of your car, remove the right front wheel, undo 5-6 small fastners & pull down the rear part of your fender liner. This will expose your overflow tank. I think it's held in place with 1-2 screws.

I'm inclined to think you have either a stuck thermostat (was it replaced during your recent service?) or a failing radiator cap that's not keeping the proper system pressure. Why all of a sudden has this popped up? Also, check to make sure all hose clamps are tight.

Good luck and keep us posted!

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo