What do fish see that they are wary of? Tag end of mono? How about the hook?

[Whitewolf]

I have long pondered. In another post it was raised would the weeless tag end of mono scare fish off the fly? Well....how about the hook itself? I believe they can be like Pavlov's repeated training. Soon they can become aware of hooks......? I have seen it too too many times a trout is 6' away and rushes to my fly when it hits the water. Then when it gets six inches close to it....or uh....when it gets within six inches of the fly....it turns and swims away. I have been told the fly did not look natural, did not have a natural drift, or was not a good "presentation". I have fished with the little round self sticking hot pink strike indicator. Have had several fish stike the indicator on the surface. I put a hook on the indicator and nary another fish hits the indicator again. Is there such a thing as an extremely hard nylon translucent hook? I tried to put a bead head on the tippet and then tie on the fly. Planning on sliding the bead head down to cover the eye and knot. The beadhead was not big enough to go over the hook eye. Probably can try again with the proper hook and bead. Anywhooooo......I have very experienced fly fishing friends that SWEAR that fish do not see the hook.......