While doing some under-the-hood stuff a few weeks ago, I too noticed that the overflow tank was empty. Since I did not have any Honda coolant, I simply added distilled water to the MIN line. Probably about 1 pint (= 1/2 quart). Since the coolant capacity is about 5 quarts, I don't think I am affecting the coolant/water ratio too bad. [I don't want to do the math.]

Back under the hood a couple days ago, and there was still fluid in the overflow tank.

I have had no cooling problems.

Beats me what this means.