But in this scenario would they not know where the ex was or, if on the run, surely they would have put out a description. JMO
Yes, unless they have nothing on this person... I think this may have been why LE has asked "Did anyone lend MH their phone that night?" They might have suspicions but no evidence.
In this case, my guess would be they need him to feel safe, so they can watch where he goes, etc. They may be investigating, and just not have enough evidence to search his residence, etc.
The interesting thing about a "she knew her abductor" theory, is that it doesn't negate any of the sightings, theories on whether she was drinking, etc that night. All those may be true and accurate. It makes me wonder what - if anything -happened six months ago as GH stated Morgan had had such a good relationship with her father for the past six months. What was wrong before then? Was there a problem with a boyfriend? This is what we don't know...
Just a theory!