While the broader technique should work for those applications, the platform (smartphone ToF sensors) probably won't. Smartphone ToF sensors have a pretty limited range. We were only able to detect cameras within 1 metre of the smartphone because we're really using the hardware for something it's not intended for.

For the applications you suggest, there are some existing military-looking devices out there that use multiple lasers to find sniper scopes, for example. My basic searching shows at least https://www.ldsystems.us/product/sniper-optics-detector/#, though I'm sure there's more.