How could the air from my res have escaped into the loop?

[Yutang13]

Rebuilding my loop (soft tube) and it’s been the build from hell. Spent most of today and yesterday working on it!

Thought I was finished, all filled and bled, but the side panel wouldn’t fit. Turns out the side fan bracket was the wrong way up.
Unscrewed it all resulting in the rad just being held up by the tubes. I hear a funny noise, thought it was a bit like air escaping. Look all around but can’t find any drips, the noise stops after a while and I carry on.
Get it all back together (not easy!) and look forward to relaxing.
Turn it on. Somethings wrong. All the air from my res has escaped and it completely full!

Now all the air is back in my rads. Can anyone explain how this could happen?

I didn’t tilt the case or anything, just caused the rad to move about and be under some stress.

Side note 1: I never unscrewed the res cap since filling it, the water level did drop a fair amount whilst bleeding. Problem?

Side note 2: I think the noise came from the top back fitting but I can’t see any water from it.

Side note 3: Yes my res is mounted below all the rads.

Please try to explain this and tell me if I should worry about this or ignore it and re-bleed.
Thanks.

 

[samcat]

Pictures please, far easier to understand with pictures!

Is res horizontal or vertical?
Tall res/Small res?
In and out on the bottom of the res or is there a return in a funny position?
Rad position relative to res?

(all this would be answered with pictures!)

 

[Yutang13]

The loop in it's current state:

The tube on the right goes into a crossflow rad that joins to the top rad at the top left. The top left is where I think the noise was coming from.
Don't know why my camera picked the leds up as purple, they're very blue.