This paper considers the use of static and mobile sensors (quadrotors) for persistent monitoring in an indoor farming scenario. The focus is on autonomous navigation of the quadrotors to monitor the coverage holes generated by a number of broken static sensors. To that end, a three-layer strategy is proposed by (i) dividing the coverage holes into several region-of-interests (ROIs); (ii) selecting the quadrotor(s) responsible for monitoring each ROI using a novel Modified Ant Colony Optimization; (iii) designing coverage control with time-varying density function for ensuring persistent monitoring. Simulation results demonstrate the efficacy of the proposed strategy in persistently minimizing the coverage holes over time.