The neighbor who shoveled your driveway before sunrise never knocked to collect the credit. Knock to give it.

Anonymous help is the purest version of the thing, and the easiest to leave unacknowledged, since nobody is standing there waiting. The person who did it clearly did not need the thanks. That does not mean the thanks is worthless; it means the value is entirely yours to create or skip. Most people skip it and then feel vaguely bad about it later.

Go and say it out loud. Not a text, not a wave from the car, an actual knock and an actual sentence. Doing that once tends to turn you into the person who picks up the shovel next time.