We don't know what happened. My baseless speculation is that NYPD have something linking him to the hostel and got a court order for the security footage. HI is an absolute madhouse, and I can't see them remembering one generically dressed guy, tipping off the authorities, and that somehow standing out amid the thousands of tips NYPD has said they're getting. Realistically this isn't HI finking, this is NYPD getting off their lazy asses and actually doing some work.
Why the fuck would you change jackets and backpacks into something vaguely similar if youâre trying to disguise yourself? Clearly the police fucked up.
I believe that this photo was purported to have been taken as many as 10 days prior, and it's also highly likely that he bought the Peak backpack just for the shooting, since it was later found abandoned.