I don't think we can rule out the boys going missing in advance of a visit with someone (whether CPS, extended family), but IMO there's a near 0% chance it was with bio mom. She hadn't seen them in 2 years, did not know the APs or where they lived, and has no legal claim to the boys since they were adopted. If I remember correctly, she heard about the 2 missing boys when there were no names or photos released yet, and just kind of put things together to figure out it was them.
Unrelated to bio mom, but I don't understand the strategy of the extended family releasing even older photos of the boys when the problem with the original photos shared by LE is that they weren't recent. Like, have you seen these little boys, this is what they looked like 2 years ago. MO(confused)O.