In this paper, we introduce the new concept of Crowd-Detectable Zero-Energy-Devices. Such devices harvest solar or ambient light energy to power themselves, backscatter ambient waves to communicate, and are detected simultaneously by a smartphone connected to the network, and the network itself, as long as the device is close to the considered smartphone. Such device is a promising sustainable solution for the future of the Internet of Things. We describe an example of use case: asset tracking with zero added energy, zero new signals and zero new infrastructure in the network, thanks to the anonymous and transparent participation of smartphones connected to the wireless network. We then present our first prototypes of such devices that backscatter TV, 4G and 5G signals, and show our first experiments in challenging conditions and environments.