What is the torque needed for chainring bolts?
A Shimano "General Operations" manual states the tightening torque for road chainwheels is 12-14 N-m.  For mountain bike chainwheels it's upped to 14-16 N-m for large and middle ring and 16-17 N-m for the smallest ring of a triple. 
The website: http://si.shimano.com/#/
The manual number I'm referring to: DM-GN0001-20-ENG.pdf 
Blue loctite or equivalent threadlocker is indicated here as well. 
I can't accurately surmise the cause of the loose and lost bolts in this specific case. However my experience with situations of loosening bolts or part failure due to "coming apart"  in various mechanical settings often comes down to incorrect or incomplete tightening of the fastener (bolt, nut, etc). Part of this process is rechecking and re-torqueing after a break-in period of normal use.