Vanishing Caliper Bolt

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Had my discs and pads changed on my Hawkeye Impreza last week by a mate on his lift (I'm real lazy)
All was fine until yesterday afternoon driving home from work I heard a knocking on the offside front wheel. Sounded like CV at first but then was happening with any slight turn on the wheel at all in either direction. Had been spanking down the motorway so thought maybe I'd blown a tyre at some point.
Got out, tyre looked fine. checked disc to make sure it hadn't become warped and knocking on the back plate. Looked fine. Was only a mile from home so drove it home still had the knocking. exceptionally bad under any sort of braking. As I reversed into my garage I noticed it making no noise and it immediately clicked. Got out and could move the caliper with my hand.
Top bolt is completely missing. There seems to be no real damage, jacked it up and had a look doesn't look stripped or anything but there is no bolt anywhere.
My guy reckon's it snapped, says he torqued it up fine and it can't have just slipped off (I'm assuming if it was rusted through he would've done something about it he's not a complete moron have had him doing my cars for 7 years and never had a problem with him)
What're the chances of it snapping? Is it possible for it to have been overtorqued/corroded not obviously? It was
What are the chances of needing new calipers?