Actually, there are news reporters who witnessed Ruby waiting about the building on the outside that day.
Please note first paragraph if this link works: Ruby lurked and waited for Oswald.
There is much more, I think Ruby actually talked with a news worker in the street that day in regards to when Oswald was being transferred or something like that. I read it some time ago but the info is out there.
That paragraph is an unsupported assertion, and it contains other misleading information.
Oswald was supposed to be transferred at 10:00. Ruby had gone downtown to wire money to one of his strippers at 11:17. Within five minutes after that he had shot Oswald.
He wasn't waiting outside. He would have gotten there too late if Oswald had been transferred as planned. He would have been too late if Oswald himself had not delayed to get his sweater.