Missing all sitemaps in GWMT since last checked (few days ago)
9:35 am on Feb 25, 2013 (gmt 0)
Still seeing sitemaps over here.
11:45 am on Feb 25, 2013 (gmt 0)
If somebody else from your website added the sitemaps, make sure you are looking at the "All" tab rather than the "By me" tab. I'm not sure why Google breaks it out like this, but we thought we lost our sitemaps for a while until we realized we were looking at our webmaster tools through a different account than normal.