Audio forensics based on fluctuations in the electrical network frequency (ENF) has become one of the major approaches for the authentication of digital audio recordings. Yet little is known about the circumstances and preconditions under which battery-powered devices leave ENF artifacts in their recordings. Our study with multiple mobile recording devices confirms the hypothesis that background noise, generated by mains-powered electronic devices in proximity to the recording device, is a carrier of ENF artifacts. Experiments in an indoor setting suggest a very high robustness and indicate the presence of ENF artifacts even multiple rooms apart from the noise source.