Coolant overflow
 

Recently I opened my hood to find coolant leaked (and dried) on the outside of my overflow reservoir. Seeing that the level inside was now low, I dutifully added coolant. I assumed it wouldn't happen again... a few days later I opened my hood up again to see the same problem.

The gauge doesn't ever seem to read high, mostly midlevel, with an occasionally jumpy needle that returns to norm with a tap. Any ideas? If I am not seeing any problems, should I just go with it and keep a lower level? Any ideas what would cause this?

Also I can't tell if it blew by my cap or my coolant level meter.

And for the basics, it is an eta, in the northeast at sea level.

Ideas?