My guess, based on posting history - the bags with red marks had approved firearms cases, with all appropriate paperwork, inside. IIRC, from a correction someone posted about one of my thoughts in another thread:

1. Firearms cases are permitted inside checked luggage, as long as they are appropriately secured and the process has been completed for notifying the airline of the contents.
2. Bags containing a weapons case are not to be identified in any particular outwardly visible fashion (Section 302 of the Brady Law).