Unity Alerts - Do they auto-delete?

I've got a couple of Unity test boxes (300 and 300F) in a lab facility and I've noticed the Alerts seem to clear down by themselves. Some have been acknowledged and some have not, so this doesn't seem to make a difference.

I've looked in the online help and various manuals, but it doesn't say anything that would lead me to believe alerts clear down after a week (which is what seems to be happening).

Can someone confirm whether this behavior is expected? And whether there is an option to extend or reduce the time in which Alerts auto delete? Or point me at the manual with this info in.

I'm running 4.1.0 upgraded last week, I'm not sure if it behaved this way on 4.0.1 as I wasn't exploring alerts at that point.

I'm also looking at using the REST API as the method of alerting into a central alerting tool, is anyone else doing this, I've figured out the correct syntax to capture the alerts, but I was wondering if anyone else had successfully adopted this approach (instead of SNMP or SMTP)?

Thanks!