The Alleged man that supposedly had a side relationship with 19 yo Leah Croucher, was said to be 27, (8 years her senior), worked at the same firm as Leah, (but in a more higher role), of Asian descent, and a practicing muslim.
Adultery is deeply frowned upon in Islam at all levels. If the relationship started going south I have no doubt he would have pulled the plug before she did. I agree with Gimmeblue, it looks like she has been silenced in some way.
Mr X has to be a person of interest (POI) at this point, all things considered. Either he knows where she is, or she is being held captive. If she had committed suicide she would have been found by now.
If she is in captivity, I wonder if someone in MK or further afield is running a double life on the side. Like the Fritzl case, or Jaycee Lee Dugard case.
We must not give up until she is found.