Exactly. If he brought curtain rods into the building that morning, then he will have established very soon after the assassination that they were missing. 2 + 2 = 4. I'm outta here.
And bear in mind: he knows who set him up----------it was the person who tricked him into bringing curtain rods to work that morning. This (so he thinks) will all come out at trial.

If it's the "person who tricked him into bringing curtain rods to work that morning..... Then he's the culprit. Because He knew that the script called for him to go to Irving to see his family one last time before embarking on his sojourn ( mission) to Cuba.... He knew the script called for him to carry something that could later be identified as a gun in a paper sack..... Thus he carried the curtain rods and his lunch in the long ( 27 inches) FLIMSY PAPER sack......
He wanted BWF to see the sack but he didn't want all of the employees gathered on the first floor waiting to start work to see that long paper sack......Thus he slipped the paper sack between the boards of the loading dock before entering the TSBD.
It's obvious that others knew what the script for the stage play called for because even before BWF reported that Lee was carrying a long paper sack the police were telling the reporters that they believed that Lee Harrrrrrvey Ossssssswald (Booooo! Hissss!) had smuggled the gun into the building in a paper sack....