We investigate time-dependent spectra of the intermittent resonance fluorescence of a single, laser-driven, three-level atom due to electron shelving. After a quasistationary state of the strong transition, a slow decay due to shelving leads to the steady state of the three-level system. The long-term stationary spectrum consists of a coherent peak, an incoherent Mollow-like structure, and a very narrow incoherent peak at the laser frequency. We find that in the ensemble-average dynamics, the narrow peak emerges during the slow decay regime, after the Mollow spectrum has stabilized but well before an average dark time has passed. The coherent peak, being a steady-state feature, is absent during the time evolution of the spectrum.