I was with you on this for awhile, but as time went on I questioned the reason they didn’t respond. Given the time line, it IS possible he came up with a story about why he was home without Gabby but had her plan. Who knows what that story was. Maybe they thought it was odd, but they didn’t think she was dead. Then her family reaches out to them. Something clicked for the Laundries, they asked him about it, called their attorney wondering what the next move should be and how he should turn himself in or if he should. It is absolutely reasonable to want your attorney present given the legal implications of turning one’s self in.
In the meantime while this is all happening Brian left. Then they waited a couple days to report him missing for whatever reason.