Dexcom uses CGM readings 1-12 to designate different errors that you only see as ???. While these will only show ??? on the Dex, on your uploader, or on your Nightscout site, or your Pebble watch they will show a number. Don’t panic, this is what those numbers mean.
And here’s how those icons relate in the official Dexcom User’s Guide
And how those arrows are calculated (mmol version for now.)