Coolant leaking from my clio '00 but now it seems fine. what happened?

I was getting it ready for mot and revving it stationary at 2k in order to clear the carbon from the exhaust system with cataclean (no insurance and tax to take to drive it)

I realise this means I lacked air cooling but I kept an eye on the temp gauge and it sat right in the middle.which I believe is optimum temp, I wasn't paying attention to the fans etc and there was no evidence of it over heating then all of a sudden the warning symbol came on and I heard it pissing out from my car whilst the temp gauge was still in the middle.

I put some new coolant in it and ran it for a good length of time, the fans wouldn't even kick in at idle so I had to rev it again and I had no problems this time and I also took the bumper off and looked for split hoses etc and found nothing.

Here's what I think happened;

The coolant was old or poorly mixed so it turned into steam (which expands by a multiple of thousands) this meant there was too much pressure and it leaked out of some sort of relief system or it was enough pressure to escape from a hose connection.

Can someone more experience verify this and explain in more detail, does a relief system even exist?

I would like to understand as much as possible about the cooling system and I'd also like to verify that its ok so when I sell it I won't get an angry buyer with a dead rad.

Thank you