IMO, it’s pretty clear what happened. Before it is known that SM is missing, multiple people that are connected to BM on Facebook but not SM, start receiving notifications that they should connect with SM.
For the algorithm to make that suggestion, something needed to happen for it to connect SM’s profile to BM’s contacts.
Facebook states this:
Help Center
What that link is saying is that any device you use to log into your profile on the Facebook app and separately Facebook messenger app, the apps will be allowed access to all of the contacts you have saved on that device.
The algorithm then takes in these new contacts and makes suggestions on connecting.
So if BM logs into to Facebook or Facebook messenger as SM on BM’s phone, SM’s profile then reads all of BM’s contacts (since it’s his device) and for any of those contacts that SM is not connected to, it suggests that they connect.
Thus why you have all of BM’s contacts receiving notifications to connect with SM before she is even reported missing.