My latest theory....
1. Terry enjoys the thrill of having a "secret" and possibly forbidden relationship (proof: per DY, her relationship with KH prior to Kyron's birth, per law enforcement her relationship with the alledged MFH landscaper took a similar form as her relationship with MC which we can surmise meant the use of text messages and since she was married at the time, presumably this meant it was carried on in secret, and possibly even her relationship with MC would fall under this category)
2. She could have been engaged in a similar relationship with an unknown party. Her communication with him likely was technology based and could have included methods more difficult to trace than cell phone texts, ie facebook chats, google chat, etc. Perhaps he shares her love of technology and had the know-how to keep their (adulterous) relationship a secret, presumably from KH but, in the end, also from LE.
3. This person did not understand that she was playing a game but desired a "real" relationship. This would not be unreasonable since usually people like Terry target easy marks. (b/c otherwise they won't be stupid enough to believe her) So perhaps this guy was emotionally needy to the point of instability and as a result of her manipulations became emotionally invested in the relationship.
4. Terry refuses to leave KH for this person and gives as her excuse her "wonderful" relationship with Kyron... she might have kept custody of Baby K (especially if KH was as terrible as she was likely portraying him to be to her mystery man) but she would have had no claim whatsoever to Kyron and, in fact, it would have been believable if she told her mystery man that if they split, Kyron would go to Metford to live with DY. Likely, if TH was trying to make herself look better and/or appear to be more of a martyr in order to garner a sympathetic reaction from her mystery man, she would have portrayed DY and TY as bad parents. Basically, she had weaved a web of lies upon which the entire virtual relationship was based and she used Kyron as the reason why she couldn't/wouldn't change her current situation.
5. Mystery man decided to "force" the issue by abducting Kyron. Perhaps he had figured out that she was playing him and snapped, maybe he had past experience with women leading him on. Perhaps Kyron had met him, maybe he was at the Science Fair... maybe he's another parent even..... Maybe the media coverage after the event made it clear to him that she had been untruthful to him. Likely he has completely disappeared.
6. TH doesn't know exactly what happened, perhaps has had little or no contact with him since 6/4 (I think perhaps she did get contact from him directly after the abduction, causing her to send up a midday SOS call to DS, perhaps the two of them were driving around looking for Kyron and his abductor) Now, she doesn't want to reveal his identity in hopes that Kyron is still be alive but won't be if she tells. Maybe the whole cellphone thing is an attempt to find a way to contact him without the notice of law enforcement. Maybe it is her hope that mystery man will eventually surface with Kyron, since his intent was to hurt HER not him. She would also be terrified that if he HAS hurt Kyron that she will go to prison and will never see Baby K again. As it is now, she has *hope* that eventually she could get visitation/custody rights, but knows that now is not the time to be pursuing them. Perhaps right now she thinks she's relatively safe in that they can't prove she did anything with Kyron, because she didn't.
This scenario doesn't negate her image as a loving stepmother (she could still have been, to a point, and might never have hurt Kyron herself or ever even wanted him to be hurt) it doesn't require a random stranger abduction, it is in line with her failed polygraphs (signs of being evasive), it explains the driving around she did as well as the behavior of DS and it makes her cellphone acquiring controversy almost logical.
Its a theory.... one of many, I suppose, and it has plenty of holes..... but its just the result of my most recent pondering.... my attempts to make the random puzzle pieces fit together.