I rather think that everything has a 50% miss chance when firing at things out of LOS, as even with the arena's current allowance for enemies to see the second 'you' and fire at the real you, they don't hit nearly as often as they do when shooting at you from within LOS.
I'm not sure if this is a good example and actual evidence. On these arena levels you're moving around a lot (whether to run away or to find a target), which increases your dodge rate. Also, you're likely to be doing this on Coward, no matter your strategy while fighting, because you have no target anyway. So 50% miss rate or not, you're bound to be hit a lot less often than when you're on a normal level, standing and shooting. Actually, with the 50% miss rate, coward tactics, long ranges and moving around, you're likely to hardly ever be hit, which isn't likely to be the case on these levels, where players die so very often :)
From my experience, LOS is asymmetric, and should be the only reason for why some monsters fire at you from what seems like out of LOS (AFAIK, this never happens in open spaces, where LOS boundries are the only reason for being out of LOS). This makes most sense where it has been complained about most: on maze levels, chances are very good that what Rabiat described on his last paragraph will happen, i.e. you'll have to vacuum clean a hallway, and somewhere on the path something will see you when you can't see it and'll take that "blind" shot at you.
There is a slight asymmetry in the way the targeting cursor works. However, it has nothing to do with who is using it AFAICT, and everything to do with where the user is standing and where they're aiming. Try pointing at a square two squares down and one to the right, then moving to the place you just targeted and pointing at the square you just moved from. Notice a difference in the choice of the middle square? It's rather annoying, but fortunately it's possible most of the time to overcome it by simply aiming at a slightly different position (instead of double 'f'ing) so as to avoid walls/barrels.
Targeting corrections work fine with targeting, as you have mostly full control over the intended target. But that's exactly the problem with LOS - you get what the algorithm gives you, and you have to live with it. Any case where you can't hit your target, directly or indirectly, is likely to be a case where you can't see it. But just like this doesn't mean that your target can't hit you, it doesn't mean he can't see you, either. It gets real bad when you can't see him due to algorithm behaviour (and not too short a LOS), but do have line-of-fire. Therefore, I doubt the same algorithm is used for both, although it seems they have the same type of quirks.
Also, I'm sure you've all encountered a monster blowing itself to bits by hitting a barrel. I'm almost certain this happens for the smae reason the targeting line sometimes shows green when it should be red and vice versa. I.e. sometimes an imp fires at you, hitting a barrel even though its AI normally refuses to fire without having a clear shot. My guess is that as far as the AI knew, it did.