Replace the question mark with a number

Two golfing friends were about to tee off, when one fellow noticed that his partner had but one golf ball.
"Don't you have at least one other ball?", he asked.
"Nope, I only need one ball."
"Are you sure? What happens if you lose that one?"
"This is a very special golf ball. You can't lose it, so I don't need another one."
"What do you mean you can't lose it! What happens if you slice your shot and the ball goes in the lake?"
"That's okay, this special golf ball senses when it's under water and it puts out a steam of bubbles. I'll be able to retrieve it. You can't lose this ball"
"Well what happens if you hit it into the long rough?"
"No problem, you see, this ball can detect the long grass and it sends up puffs of fluorescent smoke. I'll be able to see it easily. You can't lose this ball"
Exasperated, the friend asks, "Okay. Let's say our game goes late, the sun goes down, and you hit your ball deep into the trees and it gets lost among the bushes and shrubs?. What are you going to do then?"
"That's okay too. You see, this special ball can sense the darkness and it makes a beeping sound. I'll be able to get it back - no problem."
Finally satisfied that he needs only the one amazing golf ball, the friend asks, "Hey, where did you get a golf ball like that anyway?"
"I found it."