Th vast, vast majority of manhole covers are NOT welded or bolted down. They just aren't. Sheer weight and the fit of the cover in the casting ring are more than enough to keep the cover in place in most conditions.
As an example, my employer has over 7,500 manhole covers of various types in our system (which are only for stormwater runoff purposes). Less than 2% of them are welded or bolted down. That ones that are done are for either security or for the possibility of pressurization.
Manhole covers go flying when there is some over pressurization in the system. The ones I have dealt with are caused by hydraulic transients from stormwater in a system not designed for it. (essentially think of water hammer in your home plumbing but in large concrete pipes generally not designed for pressure flow). Other causes are usually related to some type of explosion. Explosions caused by someone illegally disposing of flammable chemicals can and do happen. Other times natural gas can leak into the system. Lots of potentials, and without knowing the particular, who knows.
Other than that, if the cover did become dislodged and something hit it with enough force to send it flying, it would most likely show damage from said impact.
Because the covers are recessed in their casting rings, we have not had any issues with snowplows "sending them flying". Instead, the casting ring usually catches the plow and causes it to trip. Beehive style castings that stick up from their casting are another story though...