Were the employees, sworn to secracy

[SSlater]

There had to be "Operating Rules" in the telegraph office, which we can't ignore.
Everyone in that office provided their loyalty to Stanton - first, last and always. He was the Boss.
They all knew that Lincoln issued the order to "Let the elephant run". They all knew that Stanton ignored those orders and developed a pan of his own (to keep Thompson in JAIL)

I'm positive that if anyone knew the details of Booth's plans, their reaction would have been different. All they had to work with was - Eckert was asked to go to a stupid play at Ford's theater, with Lincoln, who had all the Police protection, or Army Protection, he wanted. What difference would it make if Eckert stayed at work? NONE.
I would guess that the employees were sworn to secrecy, so I do not expect that we will find a complete and accurate description on that day. They would keep quiet, even for their own protection. They might be charged with participation in the assassination, if all was known.
In closing, late in the day Stanton learned that Thompson had been released, and Stanton found out that it was too late to allow Eckert to join the theater party
I guess we know everything that we will ever know, about that day.