The secondary (safety catch) is spring loaded - not mechanically engaged like the old type - and the spring gets lazy so it does not always come across far enough, leaving the bonnet liable to come 'loose'.
It is like this because of a failure with the primary catch which is the hook that hangs down from the bonnet to engage the lock in the bodywork. What you can't see, because it is under the soundproofing, is that this hook is riveted to the inner skin of the bonnet. This skin is aluminium and it corrodes away so the hook is no longer firmly attached to the bonnet. When you press the bonnet to close it you get a 'bouncy' feel with no positive 'clunk'. This is because you are simply bending the hook and not engaging the catch.
At 40mph there is a big bang and your road view disappears. You can see under the bonnet and the windscreen doesn't break so it isn't a disaster, but it scares the crap out of you. You can repair the hook mount :
