We did, nothing found.
Would the fluid level in the plastic radiator overflow container drop? What other symptoms would there be?
It depends on how much is leaking. So far, yours didn't sound like much so you may not see a huge difference in the level. Keep an eye on it and see if it drops over time.
Another symptom is going to be fluctuating coolant temps like when you get an air bubble in the system. The more that leaks out, the more obvious it will be.