A reminder appears on your phone and you swipe it away before seeing which app sent it. On a Pixel, open Settings, then Notifications and Notification history. If history was already enabled, look for the app and time in the list. Pixel keeps dismissed alerts for the last 24 hours. The option and its location can differ on other Android phones.
Try it with a harmless reminder
If history was off, enable it if you want the phone to retain future alerts. A notification dismissed before you enabled it will not appear. Make a reminder called “Check the timetable,” let it arrive, then dismiss it. Reopen history and check whether the app and time are listed.
Open the app to follow up
Tapping an entry can open the app that sent it. Check the current item there: a notification may describe a state that has already changed. If your phone is shared, decide whether keeping recent notification previews suits how you use it.