I bought a Marshall Origin 50C 7 months ago. Good sound, but too many watts to play at home, I used it in the rehearsal room maybe 4 or 5 times, but due to the pandemic the rest of the time the amp has been at home practically unused.

A few weeks ago I turned it back on for the first time in a long time and found that it makes a noise (buzzing) even without connecting the guitar cable.

I thought it could be due to humidity from the winter, so I left it on for an hour or two, hoping the heat from the valves would help dissipating the humidity and eliminate the hum.

The noise didn't go away, so I put the amp back.

When I looked back a few days ago, I found a weird stain around the power switch.

The 'stain' actually is that the shallow golden part has disappeared around the amp power switch.

I haven't had time to naturally 'scratch' the surface with my fingers when turning the amp on and off, as in total I will have used the amp maybe 15 times since I bought it.

I have never seen this in any amplifier, especially with such a short time of use and a prestigious brand such as Marshall.

Well, maybe I was unlucky and got a faulty model, because it is the third Marshall amp I have owned and the first two went well, but I wonder what happens and what I can do to fix it.