The purpose of this study is to evaluate the efficacy and safety of a wearable device in detecting previously undiagnosed atrial fibrillation through non-invasive long-term monitoring using a wearable single-lead ECG patch (MEMO Patch M, MEMO Patch2) in 100 patients with Embolic Stroke of Undetermined Source (ESUS) who require atrial fibrillation screening. Participants who meet all inclusion and exclusion criteria and provide written informed consent will wear the MEMO Patch M and undergo in-hospital telemetry monitoring using MEMO Cue for a minimum of 12 hours and up to 8 days. During hospitalization, 24-hour Holter monitoring will also be conducted. Before discharge, the investigator will review these results to determine whether implantable loop recorder (ILR) implantation is necessary. At the time of discharge, participants will wear MEMO Patch2 for up to 14 days for the first ECG monitoring period. One month after discharge, the investigator will review the results of the first monitoring and initiate the second ECG monitoring with MEMO Patch2. Three months after discharge, the results of the second ECG monitoring will be reviewed, and participation in the clinical trial will be completed.