On Friday, 4 June 2021 at 15:35:24 UTC+1, wrote:
No, we've been here since they were put in and there were no holes to fill up with dirt. Hubby thinks they went in from the top and the fibres closed over them really well.

I'd be inclined to agree. Many decking screws - such as Spax Wirox decking screws and similar designs - have relatively small heads as they rely more on an upper reverse-thread to hold the board down. If they were sunk a bit deep it wouldn't surprise me if the surrounding timber pushed to one side returns to cover the heads after a few weather cycles.