1. These alerts will pop up when the data source resources stop providing metrics, which can be a VM getting powered off/becoming unregistered/unavailable for monitoring or otherwise not be able to report performance metrics. The Custom UI mainly how I manage them, or if you were very motivated you could do a postgres query. I expect this will change in future major version releases.

2. You can view the suppressed or suspended alerts in the Custom UI Alerts Overview. There is a filter for this category of alerts at the top