Your ex and you need to work together to resolve this situation. There are both legal and practical issues here, and the police identified only the practical one: a 16 year old will 'vote with her feet' unless you address the underlying issues.
I would advise against trying to 'force' things and find out what is really going on here. What is your daughter running away from? Are there underlying psychological issues? Is she being manipulated or in some way threatened by the boyfriend? Is there something else we're both missing?
There are LOTS of other steps you need to go through before anyone could suggest the next best course of action. Please consult with a local family law attorney to determine what you should be asking and doing at this point! Good luck.