Background and Clinical Significance: The widespread use of cardiac implantable electronic devices (CIEDs) has led to an increasing number of pocket-related complications. While early hematoma and infection are common, late-onset non-infective cystic pocket collections are rare and may closely mimic infectious processes, creating significant diagnostic and therapeutic dilemmas. Case summary: We report the case of an 81-year-old woman with a complex medical history (including a previous device-related infection requiring complete pacing system extraction), with a gradually enlarging, painless elastic mass at the secondary pacemaker pocket site, without local inflammatory signs, with normal inflammatory markers, and repeated negative blood cultures. Imaging demonstrated a large cystic collection surrounding the pacemaker. Surgical exploration was performed and a large volume of odourless, thick, yellowish fluid was evacuated from a fibrous capsule. Complete extraction of the pacing system was undertaken. Fluid microbiological cultures were negative and histopathological analysis revealed a chronic, nonspecific fibro-inflammatory granulomatous process. A leadless transcatheter pacemaker was implanted with favourable clinical and procedural outcomes. Conclusions: This case highlights the complexity of decision-making in a late-onset large fluid accumulation within the device pocket when clinical and microbiological evidence of infection is lacking, and the etiology remains uncertain. The local tissue findings, after the pocket has been opened and drained, along with the distinct clinical characteristics of each case, must ultimately define the therapeutic strategy. In high-risk patients with diagnostic uncertainty, leadless pacing systems provide an effective strategy for definitive rhythm management while minimising the risk of future infection.