Enemies have a smaller sight radius than the player, but their max attack range is farther than the players. You may have noticed that the player can only see 14 squares away, while some weapons have a range farther than that, yet you can only shoot a target up to your sight range (except immobile targets like explosive barrels I think). This is intentional on styg's part and it's supposed to be a kind of balance, also it's supposed to make it so being shot by a sniper feels like being shot by a sniper in that you can't actually see them shooting you.
So, you'll never be attacked by anybody that was never in your sight range to begin with; if you're being sniped, that person has been within your sight radius before at some point (may have been in stealth though).