That one is a mystery. If you aren't losing fluid from the radiator, and your carpet is dry, then you don't have a leak. Here is what I dug up from a Popular Mechanics article. It has to be leaking somewhere for it to smell like maple syrup though. The only thing we haven't checked was the intake manifold. Check on top of the engine carefully for signs of antifreeze. It could also be a head gasket is leaking. I would get it to a mechanic ASAP.
1) SMELLS LIKE: MAPLE SYRUP
WHEN: After the engine has warmed or possibly even after it's shut off for a few minutes.
THE CULPRIT: Coolant containing sweet-smelling (but toxic) ethylene glycol is leaking from somewhere. It could be coming from a radiator or heater hose, a failed intake manifold gasket or cylinder head. It might be coming from a leaky radiator cap or the radiator itself, especially if you smell it outside the car. A strong odor inside the passenger compartment probably means a bad heater core.