Re: Small leak under engine
i don't know the specifics of your particular vehicle, but if you topped up the tank and did not drive, then it seems very unlikely that it is your thermostat. this is the part that regulates the flow of coolant around your engine, and if the engine did not run, but you still lost coolant, i can't see how it could be the thermostat.
if it is leaking without the engine running, it is more likely a split pipe somewhere.
hope this helps, matt