Here they have a PIC running all the time and processing the signal from a piezo. They don't say, however, how long the battery will last.
http://talkingelectronics.com/projects/Elektor/Whistle/Whistle-1.html

 

This one could actually work as it's in a printed book. The only IO they use is 4069. The question is whether it can be easily adjusted to respond to higher frequencies.

 

A piezo tranducer is very sensitive as a microphone to high audio frequencies near its resonance of about 4kHz. But people cannot whistle at such a high frequency.

My daughter bought a keys finder at the Dollar Store then asked if I can fix it. She said it beeps whenever she speaks and when the TV is on. I whistled at it but could not make it beep. Inside it had a black blob of an IC and not much else so there was nothing I could do to make it work properly.

 

But how much could the rest of the circuit consume?
Also, I found only an electret microphone for 40Hz–20kHz. Do you think that is enough?

Chances are the frequency response of that mike extends way beyond 20 KHz, they just specify numbers that the audio crowd can relate to.

When you're young you can usually only hear up to 17 KHz and that goes down on a steady basis with age - even faster if you're exposed to any sort of decent noise levels during your life. At 55 I probably can't hear much above 12 KHz especially because I've had a few jobs that involved noisy equipment and also enjoyed rock concerts. Your hearing also starts to distort a bit as you age.

I can't imagine what the hearing's going to be like for some of the recent generation that plant those 1,000 watt amps in their trunks.

I'm somewhat of a purist with high end audio equipment and my limited hearing doesn't bother me. Anything below 50 HZ you feel more than you hear and aside from the overtones from cymbals there's not much to be heard above 8 KHz either.

 

I like the sizzle of high frequency harmonics of music and the consonants (breath sounds) of speech from 8kHz to 20kHz. Without the high frequencies then everything sound bad like an AM radio or a telephone.

 

I like the sizzle of high frequency harmonics of music and the consonants (breath sounds) of speech from 8kHz to 20kHz. Without the high frequencies then everything sound bad like an AM radio or a telephone.

Enjoy it while you can and take good car of those ears. I haven't really tested myself in years with an audio oscillator, just estimating. What bothers me most is the slight amount of distortion I hear that I know isn't there. Luckily, over time, you can kind of train that out of your mind. Most people never even notice their hearing as it degenerates with age, mine took a rather noticeable jump shortly after a date wanted to dance right by the speakers of one of the nastiest sounding local bands there is around here.