<p>The failure to forget a completed prospective memory intention, which is known as an aftereffect of prospective memory (PM), can have a negative impact on individuals. The underlying mechanism of this phenomenon—whether it stems from a resource-dependent top-down monitoring process or a resource-independent bottom-up spontaneous retrieval process—remains unclear. The focality of PM cues refers to the classification of a PM task as either “focal” or “non-focal”—a key factor that may influence whether monitoring is required for processing. This study employed event-related potential (ERP) technology to investigate the activation state and temporal dynamics of intentions following the completion of a PM task. By analyzing the influence of cue focality on PM aftereffects, we aimed to evaluate the strategic monitoring and automatic processing theories. Consistency between behavioral and ERP findings revealed that completed PM intentions remain activated (resulting in aftereffects) rather than being deactivated. Nonetheless, the behavioral and ERP data did not show differential impacts of focal versus non-focal cues on PM aftereffects. Both experimental groups demonstrated a significant number of commission errors and ongoing task interference compared to the control group. Additionally, an enhanced amplitude of the prospective positive wave—associated with PM cue retrieval—was observed in the parieto-occipital region during the 400–600 ms post-stimulus interval. These findings suggest that PM aftereffects are underpinned by a strategic monitoring process that requires attentional resources, irrespective of the focality of PM cues.</p>