In this letter, we present a swarm robotics control and coordination approach that can be used for locating a moving target or source in a GNSS-denied indoor setting. The approach is completely onboard and can be deployed on nano-drones such as the Crazyflies. The swarm acts on a simple set of rules to identify and trail a dynamically changing source gradient. To validate the effectiveness of our approach, we conduct experiments to detect the maxima of the dynamic gradient, which was implemented with a set of lights turned on and off with a time-varying pattern. Additionally, we introduce also a minimalistic fully onboard obstacle avoidance method, and assess the flexibility of our method by introducing an obstacle into the environment. The strategies rely on local interactions among UAVs, and the sensing of the source happens only at the individual level and is scalar, making it a viable option for UAVs with limited capabilities. Our method is adaptable to other swarm platforms with only minor parameter adjustments. Our findings demonstrate the potential of this approach as a flexible solution to tackle such tasks in constrained GNSS-denied indoor environments successfully.