A system for detecting arrhythmias in cardiac activity, comprising: an implantable cardiac monitoring device configured to detect atrial fibrillation utilizing an on-board R-R interval irregularity process to analyze cardiac activity signals; a memory comprised in the ICM and configured to store specific executable instructions; and one or more processors comprised in the ICM configured, when executing the specific executable instructions to: obtain a CA data set that includes CA signals for a series of cardiac events and that includes device documented markers within the series of cardiac events, the device documented markers being indicative of AF detected by the ICM; apply a feature enhancement function to the CA signals to form modified CA signals with enhanced sinus features; analyze the enhanced sinus features in the modified CA signals, utilizing a confirmatory feature detection process (137, 307), to identify false AF detection by the ORI process; and record a result of the analysis, identifying false AF detection by the ORI process, characterized in that the one or more processors are configured to, as part of the feature detection process (137, 307): identify confirmatory feature markers from the modified CA signals; compare the device documented markers to the confirmatory feature markers to determine whether confirmatory feature markers match or differ from the device documented markers; and identify the false AF detection based on the comparison. an implantable cardiac monitoring device configured to detect atrial fibrillation utilizing an on-board R-R interval irregularity process to analyze cardiac activity signals; a memory comprised in the ICM and configured to store specific executable instructions; and one or more processors comprised in the ICM configured, when executing the specific executable instructions to: obtain a CA data set that includes CA signals for a series of cardiac events and that includes device documented markers within the series of cardiac events, the device documented markers being indicative of AF detected by the ICM; apply a feature enhancement function to the CA signals to form modified CA signals with enhanced sinus features; analyze the enhanced sinus features in the modified CA signals, utilizing a confirmatory feature detection process (137, 307), to identify false AF detection by the ORI process; and record a result of the analysis, identifying false AF detection by the ORI process, characterized in that the one or more processors are configured to, as part of the feature detection process (137, 307): identify confirmatory feature markers from the modified CA signals; compare the device documented markers to the confirmatory feature markers to determine whether confirmatory feature markers match or differ from the device documented markers; and identify the false AF detection based on the comparison. obtain a CA data set that includes CA signals for a series of cardiac events and that includes device documented markers within the series of cardiac events, the device documented markers being indicative of AF detected by the ICM; apply a feature enhancement function to the CA signals to form modified CA signals with enhanced sinus features; analyze the enhanced sinus features in the modified CA signals, utilizing a confirmatory feature detection process (137, 307), to identify false AF detection by the ORI process; and record a result of the analysis, identifying false AF detection by the ORI process, characterized in that the one or more processors are configured to, as part of the feature detection process (137, 307): identify confirmatory feature markers from the modified CA signals; compare the device documented markers to the confirmatory feature markers to determine whether confirmatory feature markers match or differ from the device documented markers; and identify the false AF detection based on the comparison. identify confirmatory feature markers from the modified CA signals; compare the device documented markers to the confirmatory feature markers to determine whether confirmatory feature markers match or differ from the device documented markers; and identify the false AF detection based on the comparison.