If my players fail an Awareness check I simply tell them "Something doesn't feel right" or something to that effect. This is no different than what the actual player is thinking but for some reason actually giving them that information is enough to calm players need to get the test right as they usually go investigate something anyway. If the Awareness check was just to spot something like a data-slate then the failed Awareness check has done it's job, leading the players on a goose chase as they think enemies are near. 

And am I the only person who have them roll misleading Awareness checks anyway? I have them roll Awareness ALL THE TIME for mundane things in addition to important things. Passing these mundane checks allow them to spot unusual but flavorful events that add to the atmosphere of the game, like maybe while walking through an Imperial world they witness the public stoning of a criminal. Not exactly the role Awareness was created for but players do feel rewarded when they pick up these little dashes of flavor. On the other hand Awareness is also tested to test things that are completely harmless. Once had a squad of Astartes tumbling and leaping through a jungle chasing what they later found out was just the local fauna (Like a deer, except this is WarHammer so it has like skulls and spikes all over it and can spit poison or something. A WarHammer deer.)

Minnjitsu said:

Why? Surely in this case the clue is in the name - Fate is almost by definition something out of the hands of the characters. It doesn't represent any additional *effort* on the part of the PC, it's literally good luck. You can luckily notice something just as easily as you can luckily shoot somebody.

 I agree. If anything you're basically twisting fate in your favor. So the difference between spotting the Eldar Ranger aiming his rifle at you and being oblivious to him is reasonable grounds to spend a fate point. I don't recall there being any prerequisite for the use of a Fate Point other than, in this instance, that the player is making a test and the point allows a reroll.

 Well fate is how you run it.  I run fate as "It is you fate to do this".  If I had the right circumstance I would allow a fate point on awareness (Eldar Ranger).

 RAW, fate is totally OOC. You decide that your character shouldn't suck so hard and you spend a point to attempt to do better.

Fate is nothing like, say, willpower in World of Darkness. Willpower is spent on actions your character can consciously push themselves at such as driving a car really fast in the rain or writing an essay with a deadline looming. You can't spend it retro-actively however.

I don't think anyone has said it represents that the characters are trying harder, but rather that fate itself is being manipulated in their favor (which, for all intents and purposes, is exactly what's going on since you're improving the chances of their success). The character is not conciously aware of the fate point being spent, but rather it's more like a plot device in a film where coincidences seem to line up nicely.