Okay... a related theory...
What if it was a workman who had developed an infatuation with her and instead of the weekend getaway being the catalyst... what if they were almost done working on her condo...
The locks would be changed...
He wouldn't have a reason to be around Jen anymore....
Feeling desperate...and "now or nothing" he acted.
I say this because I had read Jen and Rob traveled to see one another every weekend... so going to see Rob for the weekend wouldn't be any less upsetting (I would guess) than going away with him for the weekend.
I thought about this as well, and truthfully cannot rule out the possibility.
However, they operated with a Master Key, and we cannot determine who had access except to say it could have been really any worker given they were permitted to utilize the empty units randomly with no ID or reference checks. So the lock change would not matter.
I agree that the weekend getaway if she is staying with him most weekends, would be non-event. But that to me might amplify a reaction by someone that sees this first ever vacation weekend together as a stake through the heart, which could apply to either theory. Throw in the x at her condo for the weekend, the fact by all accounts (please correct if appropriate) she did not give permission for the party, it was supposed to be held elsewhere and that fell through, I feel compelled to ask "Who had the keys to get into her house in the first place?"
Could it be a worker that moved her car- absolutely. Could it be a worker that caused her disappearance? Possible, but I go to "unlikely" based on the sheer lack of clues, signs of struggle, missing items that would ID her, and unfortunately, the fact that her cell was "off" by 6 or 9am, depending on which report you read. Even very organized offenders with no tie to the victim previously, will not go to these lengths, sometimes even taking a momento or placing the remains in a place they have picked out long beforehand, but would have to be very close to her condo, imo, to control where and to a degree "when" the victim is found.
Someone that does NOT want the victim to ever be found because they HAVE a tie to them, does.
I truly believe that her cell and house phone records paint a more accurate portrait of what we are dealing with here. Who was calling her cell, home, or emailing her while she was away? Who called her immediately upon return?
When were the getaway arrangements made? Was there any calls to LE over the weekend re the Man-Festival at her condo?