One problem I found with Marathons is if they have punctured due to tiny pieces of glass is that after I remove the glass the punctures reoccurred. I finally worked out that the kelvar linnings had broken due to the glass penetration and if one felt very,very carefully it was the sharp end of the lining that was causing the re-puntures. Like a minute cable end pointing upwards.

I trimmed the fibres and filed it down and then had no further problems. I would always check this now if I remove any glass fragments.
The effect was very subtle, and can easily be missed with the normal tyre inspection after a flat.