I think MF came into LE's cross-hairs on Dec 18th, when the eyewitness came forward stating she saw a woman with a baby willingly get into the back light 4-door car occupied by another woman whom she had greeted (so presumably knew and trusted).
MF used her 'best friend' status to her advantage in many ways. But, I think the fact that MF was at the top of Heidi's friend's list worked against her once LE knew they were looking for a familiar woman with a light 4-door vehicle.
That's probably when the FBI Behavioral Unit was summoned to build a generic profile as it related to what was known about MF, while investigators were checking MF's vehicle records, scouring her electronic footprint, and putting together a warrant request to access MF's phone records (per Arrest Affidavit).
By the 19th.......... LE had authorization to access MF's phone records, they had confirmation that MF did indeed have a light 4-door 2015 Nissan (seen in the resident's video footage and later identified as MF's), the parking lot eyewitness had picked MF out of a photo line-up as the woman in the car, and the behavioral experts had confirmed that MF fit the bill for a woman who might be desperate to acquire a baby.
Since LE could not be sure that MF wasn't hiding Heidi and Margo in a voluntary disappearance (or holding both against their will) inside the Bo Jack residence..........I think they took the first chance they got to confront an occupant outside of the residence (to avoid a possible panic killing if mother and/or baby were still alive), and thus confronted CG exiting Target with baby supplies on Dec 19th.
All MOO, based on the facts available to us at this time.
