Well, one clue that slug from the guard's pistol after it ricocheted from the bandit's bulletproof vest.
7 seconds sound clip from Batman (1966) - Zelda the Great (S1E9) TV Series.
You can hear this line at 00:05:53 in the Blu-ray version of the movie.
Quote context
[...]
- Alfred, get ready to catch me. I think I'm gonna faint.
- Sorry about the holdup. What's the problem?
- You-you just said it yourself, Batman. A holdup. Big.
- We're on our way. To the Bat-poles.
- We hoped that it might contain identifiable fibers from the bandit's clothing.
- Our hopes were blasted. Lab report, negative.
- I'll take this anyway. Our own equipment at the Bat-cave may prove more revealing.
- Meantime, let's ponder the crook's motive.
[...]