Burned spot on my H4i humidifier?

I got a new H4i humidifier water chamber probably less than a year ago. I just noticed that it has a brown spot about the size of a quarter right in the middle of the floor of the humidifier. Does that mean it is getting burned??
I do keep the humidifier setting on almost the highest setting all the time. Seems like I need that much humidity. I don't get any rainout. I've always used distilled water, except for maybe 2 nights total in the last year.
What do you think it is?
Thanks.

I would call Resmed and see what they have to say. Did you ever sleep unintentionally without the distilled water?

Thanks Ems,
Do you mean did I ever sleep without any water in the chamber? If so, no I never did.
I did use our well water just a couple nights, but that's all. Even though its high in iron, I can't imagine that would cause this brown mark so quickly.

Burnt spots on a Hi4 humidifier indicate that there was some water on the heater plate when the tank was put onto it. The heat in contact with water causes the burnt spot. Never let any water stay on the heater plate. Dry it off before putting the tank in place.

Minerals in your rural water supply could react with the metal in the tank, staining it.
This is just one reason to always use distilled water.
Just because it says "stainless" does not mean it is the same quality as a stainless steel surgical instrument.
The tem is broad, and encompasses many levels of quality, without breaking any law.

Thanks everyone.
I very well may have had some water on the plate.
Will that burned spot cause any problems?