Its a common (almost binary) behavior in these games. They react as "all is well", or "all hell (breaks loose)"... Worse... If you use a Stealthboy during their attack, most will switch back to random gossiping like you were never there.
Spoiler and here I thought the topic was about http://i271.photobucket.com/albums/jj125/Gizmojunk/x-ray-specs-ad.jpg.
Don't worry Gizmo, I thought the topic was about cardboard X-Ray specs too. I remember waiting anxiously 6-8 weeks as a child to receive my new accessory that would give me one of Superman's powers, only to suffer one of the first cruel blows in the life long school of hard knocks. Damn comic book ads.
This is the same problem that was in Oblivion that is commonly referred to as "psychic guards". The problem stems from the fact that the game engine does not use sound or line of sight to determine if an NPC can detect your murder or assault, only an area radius, which can extend through walls and outside buildings. If an NPC is inside this radius, they automatically detect the crime, regardless of methods used or walls in-between.
There is also a binary factor like Gizmo pointed out. Sometimes a world switch is "flipped" for a faction when you kill one of their members, turning them all hostile, regardless of whether or not they should be aware of the deed.
I imagine we will see a little of this in New Vegas, as apparently kills made on in the wasteland of the Mojave will gain Fame and Infamy with different factions, regardless of the fact that if you kill a lone NCR trooper and no one else is around, your faction standings should not be changed one bit, because there are no witnesses left but yourself to report the deed.