I believe that there were rumors of a plot. Whether there actually was one or not is another matter.

Supposedly only two people knew of Lincolns exact travel plans going from Springfield, Ill. to Washington. They were Congressman Elisha Washburne and General Winfield Scott both of whom were close confidants of Lincoln.

One would wonder why such extraordinary measures would be felt to be necessary if there were not actually some sort of rumors floating about?

Would he have been taking such measure to avoid being mobbed by the crowds? That would be something that I would think Lincoln would have welcome as the newly elected President. I don't think he would have been hiding from his admirers.