Tuskers the mystic adept is paranoid that the house he and his team is about to enter is an ambush. He happens to be right . He casts a Detect Enemies spell so that would be attackers don't get the element of surprise. With 8 hits on his F8 spell he's confident that he'll find something, but he doesn't. Little does he know the team who set up the ambush expected their targets a little bit later and decided to go grab some munchies before the action went down. Tusker's and his team head on inside confident nothing is a miss. The ambush team arrives late after eating at McDuff's and the team's mage, Chiggins, approaches the building in Astral Perception just in case there are lecherous Devil rats lying in wait, concealed and hungry.

Here's where I get confused. Tusker's sustained Detect Enemies spell is still in effect. Let just say that edge of the spell's detection range is right outside the building on the side walk. Does the spell's aura extend that far? Will ambusher Chiggins and crew be able to tell someone is keeping an astral eye out inside the building? Can he assense the spell from out on the street and tell that it is a detection spell and abandon their previous plan?

Simply put, does a detection spell's aura extend for its entire detection range?

Well, aside from the fact that the excact nature of Detection spell auras is a very touchy one, the normal answer should be :
No, unless they can see the character himself astrally. The best they could see were probably something like random spark of mana in the astral, but those should be common enough. If they had the character in sight, they'd probably figure out the sparks were forming a ball around the charcter ... imagine the astral space if the spell would be more visible and a couple of mages decided to cast Extended Range Detect Whatever at the same time, all around town ...

the spell is targetted on a specific person, notwithstanding the spell detects in a large area. you would have to see the person to assense the spell.

that being said, casting a high force spell does leave an impression on the astral, which if tusker doesn't clean up, will make it obvious that someone cast a spell there, and possibly what that spell was. it would not do anything as far as telling you current location, whether the spell is still active, etc...