Button cameras are a thing, and shirt pockets are basically made for pens. I was into spy tech as a kid, and even 20+ years ago hidden camera tech was solid, and it's better now with the general miniaturization of everything. I did a quick search and $100-200 can get you a really solid hidden camera that blends into your outfit even up close. For under $50 you can get something that you wouldn't notice from across the street.

I think it's a better idea to just legally mandate a red light that's on every time the camera is. Kinda like when Japan mandated shutter sounds after creeps started taking upskirts.