  We investigate time-dependent spectra of the intermittent resonance
fluorescence of a single, laser-driven, three-level atom due to electron
shelving. After a quasi-stationary 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
