Hi mate

To be honest, it is far more likely that the bolt rattled loose and simply fell out. There is abolutely no way in hell that there is enough vacuum to shear a bolt. That said, it could have snapped, but again this is unlikely.

If you want to be sure, take the back of the plenum off (you will need a gasket) and check, otherwise, replace the bolt, and hey presto problem will be solved.

If it makes you feel any better, I had the exact same problem on mine, when a bolt fell out of the throttle elbow! All fun!

Cheers

JJ