On my wh-1000xm2, wh-1000xm3, wf-1000xm4 and lastly wf-1000xm5, there is a quite high frequency pitch (usually coincides with some public transport beeps, and some accidental squeaking of doors) that toggles ANC to transparent mode automatically. I remember reading something about this on Sony's support website.

My XM4's always do that at the beeps from the cash register, although I always attributed that to their volume rather than frequency. My theory was that they refuse to produce sound loud enough to cancel the beeps for safety reasons.

Also triggered by baby screams unfortunately