I think it is an adorable topic!!
Now..whether they used the same paper bag or not? I don't know. But you brought up an idea that I hadn't even thought of before....he does show up with a paper bag a lot!!!
Greenhouse Jungle...he has the bag with the african violet....Double Exposure...in the grocery store scene he has Kepple's tape recorder in a bag....Excercise in Fatality..he has Gene Stafford's shoes, along with his own lunch, in a bag...and I believe in Case of Immunity...he is eating his lunch outside the gates of the legation and I believe he has a folded paper bag with him...
And I am sure there are many other instances when Columbo has that magical paper bag with him...

lol...i hope it wasn't the very same bag each time. in "Now You See Him" he shows up with greasy fried chicken in the bag! i really hope he didn't reuse that one!

If he had shown up at the mission in "Negative Reaction" holding a paper bag, imagine Joyce Van Patten's reaction - "Now, now, brother, liquor isn't the answer."

hahahaha that was a good one Grant