You will always receive a message from Gaiyo if you have forgotten/missing check-outs.
If you forgot to check out, what you can do depends on the time and the carrier. Did you travel with the NS? Then you can still check out later in the day (up to 6 hours after checking in and no later than 4 a.m.) at the station. Please note, this is only possible if no other public transport transactions have taken place with the Gaiyo Card in the meantime. It is also not possible to check out at the same station where you checked in.
Do you only see later that you forgot to check out? Then you can request a refund at www.uitcheckgemist.nl.