I know this is a bit of an older thread, hopefully you found an answer. but what you should follow is that rather than judging if that individual actually noticed the hit, is apply a minor penalty for pack or hopper hits, and majors for everything else pretty much. This is the current psp practise.
The rule of thumb is its an objective test rather than subjective i.e. should that player 'ought to have known he was hit' is the question you ask. so obviously if he gets clipped on the outside of a loader, we wouldnt expect someone to notice this, but we would certainly expect someone to feel hits anywhere on their person.
Although rather interestingly, over here in euroland we practise that a hit anywhere including hopper and pack incurs the same penalty as a hit straight in the lense. interesting how us foreigners are able to feel pack hits so much better than americans