FD also stopped to buy the newspaper for FGM. (as per CO's book Missing William Tyrrell, chpt 2)
That is probably when he sent the text message. There is a paper shop in Kendall. It was mentioned in the testimony by Mr Connell - RC's nephew in law - at the inquest.
Link
The shop would probably be just a couple of minutes away from the house on Benaroon Drive, I would guess. It's a small town.
And if that is where FD stopped for the paper, there would be a witness who sold him the paper, and maybe even shop CCTV.
The police could likely also see (the side of) FD's vehicle as he returned past the tennis club CCTV.
And that may be how the police confirmed the time.
imo