I have been able to trigger it when fumbling around with adjusting the fit. If I run my finger over the top of the body I can get a brief squeal out of them. Happened just a few seconds ago. I also have noticed on multiple occasions that if I take them both out of my ears and cup them in the same hand, they squeal audibly enough to hear faintly from a couple feet away. The skin contact sensor is very sensitive.
However, unlike a lot of other commenters in this thread, I feel like the APP3 are a huge leap forward from the APP2 and have had zero regrets with the upgrade (other than the forced iOS 26 upgrade, but I feel like that's inevitable anyway.) They stay in much better, the fit is more comfortable, the battery life is better, the ANC probably drops background noise another 10db subjectively, and most of all, the sound quality is absolutely stellar. I have owned several midrange headphones and a portable DAC and I find myself preferring the AirPods over them. I haven't worn my Sennheisers since I got the APP3.
However, unlike a lot of other commenters in this thread, I feel like the APP3 are a huge leap forward from the APP2 and have had zero regrets with the upgrade (other than the forced iOS 26 upgrade, but I feel like that's inevitable anyway.) They stay in much better, the fit is more comfortable, the battery life is better, the ANC probably drops background noise another 10db subjectively, and most of all, the sound quality is absolutely stellar. I have owned several midrange headphones and a portable DAC and I find myself preferring the AirPods over them. I haven't worn my Sennheisers since I got the APP3.