Do you mean that the groups are gone or the alerts are gone? If the alerts are gone, it's possible they might have been resolved. 

both? I didn't resolve them, but maybe someone did? (I don't think so though.)

The groups aren't gone, but they're only displayed when there are alerts that are grouped to them.

The plan is to display recently resolved alerts, and then the groups would persist as long as the alerts were recently resolved, and I wrote a backend feature to return them, but I ran into a few issues:
- I'm not sure how to best present them to the user
- I'm not totally sure about my API, there's not a good spot to return Resolved/ResolvedDate, and I'm not sure if it should be a new API or the existing one.

I've got some other fires on my plate so I haven't been able to look at it further, but my plan is to chat with seanmccullough@/zhangtiff@ and get some buy in on those issues.

They currently get autoresolved when new master builds finish.  I have a change to disable that reviewed and ready to go in (https://chromium-review.googlesource.com/#/c/487084/), but it requires attentive sheriffs who actually go through and resolve everything so I haven't flipped the switch yet because I'm concerned it will just result in an unmanageable number of alerts.  I think I'd rather have autoresolved and recently closed.