Exceeding 90 days

Very soon it is going to be impossible to overstay 90 days without being noticed.

I have realised on returning to the UK that I have inadvertently overstayed in the Schengen area this year by 4 days. I have entered and departed from Spain on one trip, and entered and departed from France on the other two. Ferry for Spain, Eurotunnel for France. My passport was stamped each time. This overstay was not picked up when I left France earlier this month.
I can find no way of confessing this transgression to the French authorities which I would happily do and pay the penalty. I’m worried that it may be picked up at French border control when I return to France in a couple of weeks and that I may be refused entry.
Any thoughts/advice?

this is from January 2021 and may not be the current position in practice. The most likely consequence is the refusal of entry if the 90/180 rule has already been breached.