It's possible the agent put you on the SBY list, but then the auto-rebook or auto-cancel system did something to you for not showing on the previous flight.
I've had an agent hand me a BP (confirmed) for a flight, just to be told five (5) minutes later I'm no longer on that flight.
If there is any disruption or potential disruption to your travel plans, you really need to watch your record like a hawk with SHARES.