So the last mention of Sydney I see prior to her becoming missing was posted by her mother on this Facebook post
Facebook
The details in this broadcast post are that on August 14, 2020 the dog Macie is reported missing and a Pawboost Alert concerning the dog being lost and then found
Macie has been REUNITED! is issued . It says "Please spread the word! Macie was LOST on August 14, 2020 in Chapel Hill, NC 27510 near Behind Kings mill road - trails along Morgan creek" Later on in states the dog was missing for 5 days and 20 hours. After the dog is recovered, the mother posts she (presumably the mother) is in Charlotte. She says "sydney took her hiking.... I’m in Charlotte.. Macie chased a deer!"
What is significant about this post is that Macie the dog was reported missing the day the dog became missing and there was no delay of a few days reporting the dog missing. Also there is no mention of a concussion and on Aug 14th Sydney was well enough to go on a hike with the dog in the woods by the creek. 5 days and 20 hours later would presumably be the time frame that Sydney was either flying across the country or moving into her dorm at Berkeley, but the mother does not mention Sydney going to Berkeley. But she does mention Sydney was hiking with the dog when it ran away. The mother says she is in Charlotte so that means that apparently Sydney flew with her father to the West Coast unaccompanied by the mother who says she was in Charlotte.
Not sure why it was necessary for the mother to call Sydney out for being responsible for the dog running away in a public post on a Chapel Hill-wide Facebook group right before Sydney is flying off to college. The person in the conversation never actually asks how the dog ended up being missing in the first place. Not sure why it was at all necessary to post a public post across Chapel Hill mentioning Sydney's role in the dog running away, but it is what it is.
But reading between the lines it sounds like as Sydney was packing up to fly to Berkeley it was more than likely tension-filled because of the lost dog. And the mother did not fly with Sydney across the country and was in Charlotte.