What is the torque needed for chainring bolts? I have a bike that is no longer in production, but it has a standard drivetrain (FSA). I had some shifting issues the past couple days, and to my surprise when I did a clean/inspect, three of the 5 chainring bolts had dropped out and the other two were finger loose.

I salvaged some bolts from my donor bike. I needed a breaker attachment to my 5mm hex to get them off. There was no residual threadlock on the threads. Now I'm unsure how hard I should tighten them onto the first bike.

Why would they have come loose in the first place? How hard should I tighten them to prevent this from happening again? Would a dab of threadlock be good insurance?
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.