Since, as far as we know, KB had no plans to have dinner with PF (especially given the handoff of the child), and she was baking rolls that were left out.. It seems most likely that she was planning on spending time with someone that day... Someone that perhaps her family nor PF knew of (coworker or student) didn't seem she had many other opportunities to meet people.
That person shows up.. Is let into the home.. She probably plans to take the rolls with her and go somewhere else since she wasn't making anything else and no big meal was planned... But then something goes awry.. And she's missing.
It seems less likely PF is responsible (unless he had help and very careful planning) but that a third party we are unaware of is.. And that she was indeed on a long journey out of town (probably not voluntarily) with this person explaining the ping in ID. If the above premise is true, it would be extremely difficult to ascertain her location and whether or not she's in danger.
However... Very odd that a great amount of time passed and PF did not seem alarmed, contact authorities, or her family. Perhaps this could be explained by a more accurate description of their relationship than fiancé as the behavior seems asinine to a couple who had plans to Wed.
Just a theory.. MOO.. Limited facts available in this case.