A little-known person

[RJNorton]

and not someone we would guess.

How about Daniel Gleason as a real longshot possibility? Gleason worked at the War Department with Louis Weichmann who lived at Mary Surratt's boardinghouse. It is known that several weeks prior to the assassination Weichmann confided his suspicions to Gleason about John Surratt and his buddies acting strangely in the boardinghouse. Some sources indicate Gleason took Weichmann's wariness to Stanton but it apparently didn't go anywhere from there. Weichmann retracted his statement to Gleason a few days later, but the fears may still have stuck in Gleason's mind.

Is it conceivable Gleason was out taking a walk the night of the assassination? Could Gleason have crossed paths with detective McDevitt? Could McDevitt have mistaken Gleason for an actor? Did Gleason look anything like McCullough?

Way too many ifs here - do not put any money on it...