I then messaged my boss he told me the incident was something dodgey someone had said to me.
Out of etiquette, everyone should think about context when sending a message. Probably more so in roles of authority and those that own “terminate-able offence” compliance.
Also, people should check their paranoia. I’ve been working jobs for 30 years and never has the bad news or underhanded crap started with a message like this. Not that I don’t sometimes think this is the first time.
"I quit." "Yeah, that's what I thought you'd say."
If certain things are already in the air then just this choice of words is probably enough. I mean, in this (real) example there's no real paranoia, not the "hey.. you're fired", but still.
At a minimum they should say "this is not something you've done wrong".
* in itself very wrong, the idea that authority is there just to punish you and head teachers should be something you're afraid of.