Jean,

On May 11, 1999 Martin Shackelford wrote:

Colin:
                     
                          Bill Shelley told a group of employees that there would be
                    no more work that day. Oswald may have been nearby at the
                    time and heard this before he left the building. Other
                    employees were outside the building and didn't return until
                    later.
                     
                    Martin
                     

Where did Martin get a crazy idea like that?
Right from the Warren Commission. Supposedly Oswald told Fritz that
Shelley had told workers there would be no more work done that day.
Here is a quote from the WCR:

Warren Report: Chapter IV - Actions During and After Shooting

During the first interrogation on November 22, Fritz asked Oswald to account for himself at the time the President was shot.  Oswald told him that he ate lunch in the first-floor lunchroom and then went to the second floor for a Coke which he brought downstairs.  He acknowledged the encounter with the police officer on the second floor.  Oswald told Fritz that after lunch he went outside, talked with Foreman Bill Shelley for 5 or 10 minutes and then left for home.  He said that he left work because Bill Shelley said that there would be no more work done that day in the building.  Shelley denied seeing Oswald after 12 noon or at any time after the shooting.