Q:

How will Harry Potter be in danger if he did not return to his Aunt and Uncle's house?

I just wondered that if Harry did not return to his Aunt and Uncle's house at the end of a Hogwarts year e.g, if he went straight to the Weasley's or Granger's house at the end of the school year, will Voldemort find out where he is living?
If yes, how will he find out?

A:

Solemnity's answer, while nicely formatted, contains several errors. First, there is no "Sacrificial Protection" spell; this is an example of the HP Wikia featuring a spell that essentially someone just made up (which, of course, is not Solemnity's fault) -- I challenge anyone to find "Sacrificial Protection Spell" in the Harry Potter canon. Lily did not cast a spell -- she was murdered too fast for her to have done so.  
What we will find throughout the series is numerous references to the enchantments Lily bestowed upon Harry by sacrificing her own life in order to save Harry's. Harry is protected by his mother's enduring love. Dumbledore invoked the power of Lily's love by placing Harry with the Dursleys. In the letter Dumbledore left when Harry was an infant and was placed on the Dursleys' doorstep, Dumbledore explained what had happened to Lily and James, and implored the Dursleys to treat and raise Harry as if he were their son, which we know the Dursleys did not do. However, they allowed Harry to call number four Privet Drive his home, thus sealing and strengthening the magical enchantments already in place due to Lily's sacrifice. Dumbledore, by explaining the events surrounding the Potters' deaths and beseeching the Dursleys to take Harry in, sealed this magical protective bond, which went into place the minute Petunia accepted she would take Harry in. 

‘The magic I evoked fifteen years ago means that Harry has powerful protection while he can still call this house home. However miserable he has been here, however unwelcome, however badly treated, you have at least, grudgingly, allowed him houseroom. This magic will cease to operate the moment that Harry turns seventeen; in other words, the moment he becomes a man.
Half-Blood Prince - page 57 - Bloomsbury - chapter 3, Will and Won't

Dumbledore did not invoke any further magic. He did not make number four Privet Drive Unplottable (otherwise, how could Arabella Figg keep an eye on Harry for Dumbledore) and he did not cast Protego totalum around the house. Again, I invite anyone to find evidence of this in canon. 
Voldemort knew Harry was at number four Privet Drive, but because of Lily's protective enchantments, he was unable to touch Harry there, through most of Goblet of Fire. However, by taking Harry's blood in Goblet of Fire, it would seem that Voldemort overcame Lily's enchantments.

Harry showed them both the place where his robes were torn, and the cut beneath them.
  ‘He said my blood would make him stronger than if he’d used someone else’s,’ Harry told Dumbledore. ‘He said the protection my – my mother left in me – he’d have it, too. And he
  was right – he could touch me without hurting himself, he touched my face.’
  [...]
  ‘Very well,’ [Dumbledore] said, sitting down again. ‘Voldemort has overcome that particular barrier. Harry, continue, please.’
Goblet of Fire - page 604 - Bloomsbury - chapter 36, The Parting of the Ways

In every single book, Harry first returns to Privet Drive before heading out somewhere else, such as the Burrow. After Voldemort takes Harry's blood in Goblet of Fire, he's placed in some kind of protective custody after he returns to Privet Drive. In Order of the Phoenix the Advance Guard collects him from Privet Drive and takes him to number twelve Grimmauld Place. In Half-Blood Prince, Harry's taken to the Burrow, as he is in Deathly Hallows. He cannot go straight to the Burrow or straight to Grimmauld Place because he must first make contact with Privet Drive -- the place he calls home, even if in name only -- to keep Lily's enchantments in place. 
If he went straight to the Burrow or to Hermione's would Voldemort find out where he was living? Undoubtedly, because Lily's protective enchantments protecting Harry from detection from Voldemort would be broken and would afford him no further protection from Voldemort. As far as I can recall, it does not say in canon exactly how Voldemort would be able to find Harry once Lily's enchantments were broken. Correct me if I'm wrong, though.

A:

Slytherincess's answer is very good and thorough, but with one slight tweak. I believe the statement "Dumbledore did not invoke any further magic." is not entirely correct. 
In OotP, The Lost Prophecy chapter, Dumbledore clearly states he places a charm on Harry and "gives" Harry a shield:

“But she took you,” Dumbledore cut across him. “She may have taken you grudgingly, furiously, unwillingly, bitterly, yet still she took you, and in doing so, she sealed the charm I placed upon you. Your mother’s sacrifice made the bond of blood the strongest shield I could give you.”

One could argue that the charm Dumbledore refers to is simply the act of delivering Harry to Privet Dr. But the language he chooses makes it sound like he did a more significant magical act than simply dropping Harry off. 

