If you haven't found any leaks, and there's no coolant in the engine oil, then it's possible that whoever replaced the coolant didn't follow the correct procedure.
Failure to bleed or 'burp' the air out of the cooling system would result in exactly what you're seeing: low coolant level with not apparent cause.
I have a 2009 Kawasaki Ninja 650R with a good amount of kilometers on the clock. When I took the motorcycle out for a short ride to warm up the engine oil before changing it, the temperature light came on. The radiator, was “strangely” almost empty. I topped it off and have since put about 1000 kilometers on the motorcycle with no problems or leaks. The radiator fan comes on, so that apparently isn't an issue. Do you have any idea what could have caused the coolant loss?