From Hugh Aynesworth's account in his book "Eyewitness to History." Aynesworth was on Houston Street during the shooting and immediately rushed over to the TSBD to investigate.
Aynesworth: "I saw Brennan talking to two officers and tried to poke my nose into their conversation. "I saw him up there in the window," I heard him say as he pointed toward Oswald's sniper's nest. "No doubt he was the one. He wasn't even in much of a hurry."
One cop asked if Brennan could describe the shooter. "Of course", he answered, "I saw him real good."
Remember that at that time people were rushing to the grassy knoll and the overpass. But Brennan was telling the police otherwise.
So did Brennan get lucky when they found evidence for a shooter in the window? And when others said they saw someone in the window with a rifle? Or a rifle being pulled back?
Clearly he saw something. He wasn't making things up.