In Microsoft System Center Operations Manager 2007 and in Microsoft System Center Operations Manager 2007 R2, alerts can be raised by the monitors. Alerts that are raised by the monitors are typically the result of a state change. These alerts are displayed in the Operations console similar to the alerts that are raised by rules. However, these alerts should not be manually resolved.
For all the details see KB979388: Alerts that are raised by the monitors should not be manually resolved in Operations Manager 2007
J.C. Hornbeck | System Center Knowledge Engineer
Personally, I think the product should not allow an operator to resolve an alert that is generated by a monitor, or at least warn the operator that this alert was generated by a monitor and will not alert again while in the same state.
Also, the Command Shell script in KB979388 is the hard way of finding whether an alert had been generated by a monitor. Just select the alert and look at the details pane, and you'll see the text "Alert Monitor" or "Alert Rule", identifying whether it was a monitor or rule that generated the alert.