If your stored flight plan includes a waypoint that has disappeared from the FAA database (e.g. a VOR that has been decommissioned entirely), you will get such a message.
Just a reminder that your GNC 355 pilot's guide contains this information:
Advisory: Stored flight plan waypoint is not in current navigation database. Condition: Current navigation database no longer contains a stored flight plan waypoint. Corrective Action: Verify all stored cataloged flight plans and procedures. Modify stored flight plans as necessary to include waypoints that are in the current navigation database.
Airports that show up on ForeFlight don't always show up in the Garmin database. One is a grass strip airport I visit in the Cub on a nearby farm and the other is a neighborhood airpark.