I can tell you how it was in the pilot grievance world a few years back. It may have changed since, but if a fourth event can't be substantiated and is more than three events in a ninety-day period...you're considered an unreliable employee and fired. At the third event you will definitely get an in-office discussion.

If you're ever caught "double-dipping" (sick call coincident with guard activity), one can be terminated on the spot, although I've never seen that happen specifically.

Below these types of thresholds, "often" is quite subjective. Anything close to thresholds or consistent invites withering scrutiny.