Can you hear electronics hum? I can, especially when it's quiet. Doing hearing tests, I would hear when they turned on the sound, but not be able to hear the actual sound. Idk what it is, but pretty sure it's not ultrasonic sounds.
I can, certain alarm clocks I can hear tick or my gtx 970 had a capacitor issue that I heard for awhile before it gave up. Certain ultrasonic dog toys and whistles I can hear as well. Fun fact on things humans shouldn't technically be able to do you can also see infrared light under certain conditions.
Not everyone has the same hearing range (it tends to get worse as you get older, hence those ultrasonic teenager repellers that people claim are for cats). If a device is running at the very low end of what we consider ultrasound and OP can hear higher than most then maybe they can hear the sensors. I would be surprised if an ultrasonic sensor was running this low though.
People often believe they are hearing ultrasound, but instead are hearing clicking noise or sub-frequencies emitted from the capsule that are not actually part of the ultrasound.
sorry I can still here it.. I sometimes needs to go to the hospital for a check-up on my heart, and I can hear the ultrasound if the device is going over my head.