Yes, it's best to be drained.

I'd check the manual carefully, to make sure you have FULLY winterized it, and that there's not some pocket of water left somewhere that can cause problems.

I was thinking the other day about the infiltration of the panel. One possible good solution would be to get section of SS piano hinge and a small piece of SS sheet metal, and make a lift up panel that would simply lay across the top of the heater and extend out far enough to protect the panel. You couldn't use plastic lest it metal at some point when the top of the heater got to warm.