From memory Mrs Reid had a phone conversation with her husband and he told her the motorcade was running late. He heard it on the radio.
I believe one of the workers (Frazier?) asked Shelley if the workers could break a few minutes early to watch the motorcade. The "elevator race" was before noon, the normal scheduled time for lunch. It seems consistent with them thinking the motorcade would pass shortly after noon.
Finally found this in an oral history with Gary Mack......
Gary: When did you first learn that President Kennedy was coming to town? And then the follow-up, of course, would be when did you first realize that he was going to come right by your building?
Buell: (0:40:15) Well, I think it was a day or two before… just hearing people talk. They said that President Kennedy was going to be coming to Dallas, but that didn’t…anything didn’t register on me about that because presidents, say, visit cities in America and cities in other countries. That’s a common… a pretty common thing for them. For one reason or another, they go there for business, whether it’s political, whether campaigning, or they’re trying to be a negotiator as a… in a peace-type situation, or whatever. A president of the United States traveling to different points in the world is very common, so him coming to Dallas, no. That didn’t mean anything particular to me.
Gary: How about some of your coworkers? Was…?
Buell: Well, the morning, Friday, November the 22nd, it was, I think, around break time, they found out that… that the presidential parade was going to be coming by the Texas School Book Depository, Houston and Elm Street. And a lot of the people was very excited about that. They says, “Gosh,” says, “we got a chance to see the president.” And as they were talking, then they realized that he may come by before lunch break or after lunch break, and there’s a possibility we wouldn’t get a chance to see the parade.
So, someone had gone to Mr. Shelley and asked, “Are we going to get to see the presidential parade?” And there was a… I guess he checked with Mr. Truly, and Mr. Truly checked with his bosses who were up in… on the second floor. And they come back with the reply that, yes, we would stop and everyone could see the presidential parade. And so, we continued to work, and then at a certain time, we stopped and everyone got ready to watch the parade.