I was training Ranger, a rescue GSD to be my hearing ear dog. I had to hire a professional trainer to teach him to alert to the door, as he'd been an outside dog, and had no idea what a knock on the door, or the doorbell meant.
I have never, ever had a dog learn as fast as he did. After fifteen minutes, he had it down cold.
The trainer then asked what else I wanted to teach him, as I'd paid her for a whole hour. "How about the timer?" she suggested.
We did at the most two repetitions with the kitchen timer, then I said, "You know, I only use that timer once in a blue moon. Let's make sure he's proofed on the important stuff like the smoke alarm and alarm clock!"
So, six months later, I'm timing some fish in the oven. I have the timer right beside me, so I will be sure to hear it when it goes off. The second it does, Ranger gets up, and shoves his big head underneath my elbow. I turn the timer off, and look at him.
"I never taught you to alert to this!" I said.
I thought he must have been generalizing with all the other things I'd trained him to alert to that went 'beep'. It was several days before I remembered that one extremely brief session with the trainer!
Edit: and yes, it WAS the timer he was alerting to. I made it go off again, just to be sure!