Walls don't properly block line of sight, especially at corners. It looks like the game checks for range and if you're looking in close enough the general direction of the Enemy. Here's an example from F4: watch the small alien crawl left and gets shot (can hear the death cry) even though it's not on screen at all. Useful for F4 and F6 with the small ones liking to lurk behind blind corners/junctions. It's a time loss to look both ways and fire, but can save a run if see one go past a junction as they usually turn straight around when you get close.