Search incident to arrest typically allows you to search the person of the suspect to ensure that the person doesn’t have a weapon, or has evidence of a crime that they can destroy. Once you’ve separated a person from a closed bag, you don’t have the immediate right to search the bag; US v. Chadwick, 433 U.S. 1 (1977). OTOH, once an arrestee is actually being booked, they can perform an inventory of the contents of a bag (Illinois v. LaFayette, 462 U.S. 640 (1983)), which would have turned up the gun, etc., and it would have turned them up under controlled circumstances. But that’s not what happened here; he appears to have been arrested on a pretextual basis, and then his bag seized and searched without a warrant. However, it’s going to be up to his attorneys to make this argument, and my guess is that the state will argue that he was definitely, 100% going to be booked–despite the lack of evidence at that point to support that–and thus it was inevitable that the gun would have been found. I think that’s bullshit, but we’ll see.