Most wouldn’t, I’d imagine OP is referring to a mobile device, look at Androids dev docs they recommend sticking to 44.1khz, which we know does fail into the range of human hearing with its 22khz reproduction, albeit fewer people. I’d suspect the person being spied on would become suspicious upon many children they encounter and even more dogs fleeing from their direction.

Tested my kids - they could hear an alleged 21khz tone out of laptop speakers. The actual level of the tone doesn't matter - it was above my level of hearing. Wasn't a double blind, but they told me when it started and stopped based on a bash script with random intervals.