As others have said, that is coolant (anti-freeze). Where exactly on the "top of the radiator" was it leaking from? The cap? A hole? One of the fins near the top of the rad?
Basically for some reason the sealed coolant system has leaked some coolant.
The person above that wrote "not safe to ride" is just being cautious (and perhaps correct) - if you don't know why it's leaking (and specifically how much) I wouldn't ride it.
You say it has "magically" fixed itself... that is unlikely.
If it was my bike, I would do the following:
- Remove fairings
- Remove rad cap (make sure bike is cold first... rad cap gets hot and is pressurized).
- Check coolant level. It should be right up to the top of the rad to just where the rad cap screws on. If the coolant level is not this high, top up with coolant (premixed or a 50/50 mix of coolant and deionized water). If coolant level is adequate, or after refilling coolant, replace rad cap.
- Run bike to operating temperature (~100 Celsius, 210 Fahrenheit), while observing cooling system (radiator and hoses) for leaks, liquid, white-ish smoke (caused by coolant being leaked and burned off hot engine parts)
If it leaked/burned coolant then you have a coolant leak and you should have an idea of where it came from.
If it ran fine then you're good to go. Note that if you did have to add coolant to the rad, that you may have a small leak somewhere or an otherwise unnoticed leak, and you should keep an eye on the coolant level to be certain.
Brian
EDIT: Let me explain the logic behind these steps. I have a hunch it doesn't leak coolant anymore because the leak is near the top of the rad, and since your bike already leaked some coolant it is no longer being forced through the hole due to the lower coolant level. Topping up (or ensuring you have adequate) coolant will troubleshoot this for you. When you run the bike afterwards, it will either begin leaking again (indicating a leak in the rad near the top), some other problem will occur regarding coolant which will let you know where the leak is, or it will run fine. As I said earlier, the system is pressurized, which is why you need to run the bike to it's maximum operating temperature to see if the entire system can contain the pressure. It is no good to fix (or diagnose) a coolant leak when the bike is at 60 Celsius - the pressure is less than when the bike is at 100 Celsuis. And I don't think you really want to discover "Ahhh damn, I really DO have a coolant leak!" while sitting in rush hour traffic on the DVP when your bike does finally hit 100.
