What happens if my Non-O visa funds dip below 800,000 THB for a few hours, and how would immigration know?

I once went under 800k by 49 BAHT for a few MINUTES and had to start the process from new which included a new visa from Malaysia. Very expensive mistake.

They’ll simply check your bank statements ... it’s very easy for them .

On my Thaii Wife visa I took some money out of my account holding my 400KTHB seasoning fund, I put it back with 30 mins and it was OK with immigration, so i think it depends on the office

yep and that seasoning is 2 months date to date NOT 60 days so if your money was at 800 on Jan 1st you couldn't apply until March 1st which is actually only 59 days :O Same for the 3 months after your extension is granted for the 800K to stay in the account, it's date to date AND it has nothing to do with any 90 day reporting.

IF you use an immigration office that requires you to come back they will give you a paper when you get your yearly extension that tells you when and states the rule about it like this

I got rejected once because the date I went to renew was 1 day out from being 2 months.! saying that I have never been asked to return 3 months later.

the rules are simple IF you're using banked money for a yearly extension based on being over 50 (retirement) you need to have 800K baht in your account for 2 months before you apply, the 800K needs to stay in your account for 3 months after you get your extension and then the balance can't go below 400k baht for the rest of the year.

Some offices give you a paper that states you need to update your bank book and come back 3 months after you were granted the extension to show them you still have the 800K in the account BUT most do not.

The way they will check is they will have you bring in a year's worth of balance history when you get your next extension and look at the balance every month.

Now whether a mix up of that out/in funds thing like you outlined will mess you up or not I couldn't say BUT I'd sure move the dates of something so that you don't do that regularly.