This has been discussed quite a bit in the Prius forums. It's a pedestrian warning sound. In the Prius the little speaker is in the driver's wheel well up under the plastic liner. Its been in existence since 2012.
I can barely hear the pedestrian noise inside while driving, except when going into the garage or a drive through. When the car stop the noise stops. I do think others hear it because I've seen people look up as I drive by. As for the siren noise, yes I've heard that around 20-35 with and w/o the engine running. I think the Toyota electric motors are noisy in general and at times do make that siren noise. I think that everyone's hearing is slightly different depending our experiences in life and some frequency are more difficult to hear as we age. But yea, when I hear that particular pitch now and then so I'm looking around for flashing lights to be safe.
If you happen to buy the newer model of hybrid (2019 or 2020 Rav4 and up) there is an even more annoying sound that is heard in park, little louder when in drive up to 30Km/hr and a super annoying sound when in reverse.
Here is a video explaining the 2020 Rav4 pedestrian warning sound device and location.
All US manufacturers have to make EV (electric vehicles or hybrid) to produce a sound in Park (quiet), drive (little louder) and when in reverse (annoying) lol.
[ @Ocean Breeze; If you're going to post the exact same thing in two different topics, I'll post the same reply.]
Ermmm...This is the 4.4 Hybrid forum (2013-2018). No "beeping" when in Park, Drive, or Reverse from my 2018 R4H Limited. (Had the dealer program it to disable the in-cabin back-up beep when I had them set the Lane Change signal to 6 flashes.)