The only reason that makes sense, so far, that would cause her to run away would be if her mom was instrumental in the breakup b/w her and the 17 year old. Now, a 12 year old is thinking, "Mom made me end this relationship that really meant something to me, therefore, she must not care about me/understand me/want me to be happy." AND, if that IS the reason, then suddenly you have her in league with much older kids who would have the means and brain power to help her.
And all it takes is for her to start outlining some "abuse" or something else untoward taking place in her home life to possibly get adult help as well. If just one adult is helping her (for whatever reason) she could stay hiding out for a very long time before being found.
Add one more thing: I hate to say this, but the Chelsea King case (for example) felt urgent. If CK had been a runaway hiding out, I see an adult getting concerned and turning her in. However, there has been (from the beginning) a lack of urgency to this case that I have not understood. Whatever the reason for that is, if she's getting help from someone who honestly believes she is being abused or mistreated, I could see them thinking to just wait until some of the hype dies down and not worrying as much about the fallout. MOO