My dog was similar, but the sound he triggered on was the CRT turning off. You could sit perfectly still and press the off button the remote (so as not to give any body language signs) and he'd wake up, and wander off to the bedroom for the night.
I vividly remember when I realized it. One day in class we were watching a tape and the TV was whining the entire time. After the tape was over, the teacher started talking about what we'd seen, but didn't turn the TV off. I asked him to, because the sound was so annoying. A bunch of kids started saying they didn't hear anything, which made me think I was crazy until one other kid stuck up for me and said she heard it too.
In television sets, this high frequency is about 15 kilohertz (15.625 kHz for PAL, 15.734 kHz for NTSC), and vibrations from the transformer core caused by magnetostriction can often be heard as a high-pitched whine. [0]