Do you know if your monitoring system is really monitoring what it is supposed to be monitoring? Have you audited it? Do you have a way to be notified if it is doing the wrong thing?
In my case I found out my monitoring system at DataPipe was set up to monitor IP address, not the urls I had requested. The problem with this is while the monitoring system may show you that your server is up, it doesn't tell you if there is a DNS error that is disallowing people from accessing your site - or worse - sending them somewhere else via a man-in-the-middle DNS spoofing attack.
Monitor your monitoring systems. Audit your hosting company.