Yes, mine looks like that too. Seems like it showed up in the last few months.

Is that from your wrist or from using the band on your handlebars when on your bike?

wrist. I don't use it as my bike computer

I've had my garmin 2 years. not sure when it started. at first I thought it was dirt and sweat gunked up on it but when I started to scrape it more black showed up and I realized it was just whatever coating is on there. I'll just keep wearing it. it's not causing any issues.

Interesting. Do you wear the watch all day?

I've had a 910 for 5 years now and the back has just recently started to wear out similar to your 920. Not as much. I don't wear the watch all day. The back cover is nearly identical on a 910, with that same silver finish.

yep. I wear it all day.

Mine has more even wear on the back. I honestly thought it was dirty before I saw your post.

Had it since July or Aug of 2015.

exactly what I thought until I tried to clean it.