Why was Booth admitted into the presidential box?

[RJNorton]

I have a question about President Lincoln's security at Ford's Theatre. In some books I have read that it was normal procedure for Lincoln to attend performances with no security at all. But William H. Crook (one of the detail of White House guards) writes:

"It was the custom for the guard who accompanied the President to the theatre to remain in the little passageway outside the box – that passageway through which Booth entered. Mr. Buckingham, who was the doorkeeper at Ford’s Theatre, remembers that a chair was placed there for the guard on the evening of the 14th. Whether Parker occupied it at all I do not know – Mr. Buckingham is of the impression that he did. If he did, he left it almost immediately; for he confessed to me the next day that he went to a seat at the front of the first gallery, so that he could see the play. The door of the President’s box was shut; probably Mr. Lincoln never knew that the guard had left his post."

Was Crook right? Was it normal for a guard to be sitting in the passageway whenever Lincoln and party occupied that box?