The bag was discovered before the rifle was discovered.
Taken from reporter Ken Biffle's notes page 6 and 7:
"It didn't take the policemen long to find the cartridges by the ambush window. We all stood around staring at the brown wrapping paper found nearby. It was a reasonable conclusion that it held the rifle.
An officer in the northwest corner of the room yelled: "Over here!"
I ran over, dodging down narrow alleys in the stacks of packing crates. I was secure in the knowledge that my theory was materializing. They'd found the body of the gunman, I guessed"
Weeatherford also noted the sack
Weatherford: 11/23 affidavit
"....I came down to the 6th floor and while searching this floor, Deputy Luke Mooney said, "here are some shells". I went over to where he was and saw 3 expended rifle shells,
and a sack on the floor and a partially eaten piece of chicken on top of one of the cartons which was used as a sort of barracade, advising Mooney to preserve the scene for the Crime Lab"