So, sifting through the media coverage - including (especially?) the wonderful efforts from ppl posting in this thread...
It seems like something happened with this guy in mid-November that caused alarm with his own family and was part of the spiral that led to him allegedly shooting Brian Thompson.
His mom reports him missing to the San Francisco Police Dept. on November 18:
Luigi Mangione reported missing to SFPD weeks before shooting
The details surrounding this are still unknown but clearly she had a reason to do this.
We also know he's alleged to have arrived in NYC on November 24 (6 days later) on a bus that originated from Atlanta.
As the manhunt continues, officials say the gunman who killed Brian Thompson, 50, may have traveled to New York City from Atlanta by bus last month.
www.nbcnews.com
The fact his Mom reported him missing from SF on Nov. 18 doesn't mean that's the day he actually disappeared. But there is clearly something going on in his life at this point that made his own mother believe he was in total crisis mode - and that crisis appears to have resulted in the alleged narrative that started up in NYC on Nov. 24.