Coolant cap worked itself loose?

Further to my recent thread, post head gasket replacement with coolant level going UP and faulty coolant caps, things seemed to be going ok.
Today I was on the motorway and of course being glued to the temperature gauge, I noticed as soon as it started to go up above normal. Fortunately I was near an exit junction and was able to merge into the first lane and slow to 60 and put the cabin heater on full. As soon as I did so, the temperature dropped back down to normal. It did get to about 3/4 for a few seconds. I pulled over as soon as I could and checked the coolant tank. The level was normal but I could hear a slight hissing. I (safely) checked the coolant cap and it was very loose. I tightened it back up and drove home (about 15 miles) and everything was fine and the cap remained tight, although I only did a couple of miles at 70mph, the remainder being 50/60.

So can the coolant cap work itself loose like that? Perhaps I erred on not over-tightening it and it just needed to be tighter. Or could something else still be wrong and caused this to happen? I.e. will it happen again??

Just to add, when I got home, the coolant was just a touch low. I figured I would give it a tiny topup back to the seem as obviously some had escaped around the cap.  So I waited for it to cool down and the coolant was about another 10mm lower. I opened the cap, which hissed and the level then went almost back up to normal. I understand it expands when hot etc. I've had the car since new and always check the level, it all just seems a bit odd.Or does this sound ok?

I would do a cooling system pressure test

 

With a kit like this you 1st fit the suitable adapter in place of the coolant pressure cap and use the pump to pressurise the cooling system to @ 12 - 15 psi and look for leaks and see that the system holds pressure without the needle on the gauge dropping.
Then you use the appropriate adapter to test the cap in the same way.

I do wish this man lived near me, not only does he have a wealth of knowledge, he has the equipment to back that knowledge up.
And he has a lathe that’ll swing a greater diameter than mine. At my age that’s about it for size envy.
M

Would the cap pressure spring failing cause the cap to be loose and could it then be tightened and hold pressure again.Or would the cap be able to unwind over time, just because it wasn't tight enough in the first place.

In my experience a properly tightened cap will Not work loose. Even a failed cap should stay tight. I think the not sufficiently tightened scenario is the most likely.

I've been trying to make sure I don't still have any air in the system. When I run the engine with the coolant cap removed, the coolant level gets higher and higher until it gets near the top of the tank and I then turn the engine off before it overflows. Instructions I have seen say it should go down as air is removed. The coolant remains high until I bleed the radiator which drops the level to where it should be. Is this normal or does it mean there is a blockage or something with the radiator??? Instead of removed air, I seem to be introducing it? Could such a blockage have cause the overheating, resulting in a loose cap?

To bleed the coolant system in our cars requires the system to te bled at 3 points in  a determined sequence, to try and do it any other way is just not going to work.
this process can be found in the workshop manual or it the howto section of this site.