What happens if I leave Thailand within 7 days after my 90-day report is due without filing it?

Dave, you are incorrect on a couple things.

you are right on the fact that passport control when you stamp out of the country doesn't care if you've ever done a 90 day report, because that's not their job

You need to understand there is NO grace period for doing a 90 day report period end of story. Think of the window of opportunity to file a 90 day report as from 15 days before down until 7 days after the actual due date. DON'T think of that 7 days after as a grace period because it is not. It's only part of the window where you can file a report in person.

If you are here 90 days you are obligated to file a 90 day report, AND if you are leaving the country during the 7 days (which isn't a grace period) after your 90 day report is due you are STILL obligated to file a 90 day report.

I know someone who mistakenly thought like you mentioned, that leaving during the 7 days after the report was due meant they did not have to file a 90 day report.

When they came back the 90 day clock started counting down again, and when they went to do their subsequent 90 day report the officer saw that they missed filing a report earlier (even though they left the country during the 7 days) and they were fined 2000baht for failure to file and had to pay that fine before they could file their new 90 day report.

If you depart before the 7-days-over is up then no need for the 90-day report. Passport control at the airport won't be a bother. They just stamp you as gone.

When you return your 90-day clock starts again.

If you take 8 days then you need to do the report or face a fine.

You can do it online.....if the website is working

Ask the Thai authorities, not us.

What is the 90 day report for ? Icant see the sense in it after 10 yrs except to stuff up my day

In theory, yes. However, the chances of your local io noticing are slim. The grace period is only for those making a report, not for those leaving the country.