That's your interpretation of what the mia ping means, but more often than not is not actually how it's used, or how people expect it to be used. I am inclined to agree that that's how it should be used, but different people use it differently, and theres lots of people who rage for the lack of mia ping when someone backs and then instead of returning to lane, go for a lane gank instead. I feel like pinging mia whenever someone backs is excessive, but some people feel its necessary.
Still, apart from them being dead (which I think everyone should be aware of, not just hypothetical perfectly aware gods, considering the game announces kills), I would suggest that the lane opponents don't know (and even can't know) any of the other cases from the roaming case better than a perfectly map aware player. Remember, we're talking a perfectly map aware player, not someone who just happened to look at their minimap at a brief moment when whoever was missing, so they also know where the player(s) were (or when they weren't showing on the map) at all times leading up to that point.
If they back off into a fog of war, they could be just out of sight in the fog of war, or they could be on their way to roam somewhere.
If they went into a bush, our perfectly map aware player would see their icon move onto the bush and disappear. If the bush is attached to fog of war, then neither lane opponents or map aware player can know with certainty if they left the bush through fog of war and are roaming, or if they're still in the bush.
Sitting under their tower they'd be visible on the minimap, unless 'under their tower' is in fog of war, in which case the first explanation applies.
Recalling is probably the one the lane opponents have the best sense for, but even that could be inferred by a perfectly map aware person. If they recall in vision, then the map aware person would see their icon suddenly disappear without moving. And if they recall outside of vision (or vision is lost before the recall completes, then no one can know if they actually were recalling, or just faking out a recall (perhaps for the very purpose of prepping a roaming gank).
I suppose in that last case, champs with stealth might be difficult to distinguish from someone recalling solely from minimap appearances, but they're a special case that probably wouldn't come up much in practice (people usually don't recall in vision, and champs with stealth prepping to roam usually can't afford to use their stealth simply to leave lane, and would rather use it for the gank itself). And even then, some people feel pinging mia on recalled enemies is important anyway.
edit: I recognize that lane opponents will know the health of their lane opponents fairly accurately, and so inferring recalls rather than ganks would be reasonable in the majority of cases, but strictly speaking it's possible people might try to set up a gank despite being low health.
