Use of the internet by child sexual offenders (CSOs) poses an ongoing and significant problem for technology companies, government and law enforcement. Online engagement among CSOs is not only for the production, use and sharing of Child Sexual Abuse Material (CSAM), but also for information sharing, and discussion on offending. Pivotally, while recent progress in detection and prevention methods have been promising, a proportion of both online and offline offending goes undetected, and as a result, most of what we know about CSOs is based on detected cases of child sexual abuse, or CSAM offending. To help inform opportunities for detection and prevention, it is important to study CSO interactions and behaviours before they are detected. Our study aims to build our understanding of undetected CSOs, by examining threads and posts by self-identified CSOs on a Darknet forum. These forum threads featured information on engagement with child abuse material, methods used to avoid detection during online or offline offending, and the methods used to avoid or restrict themselves from offending. A qualitative analysis provided important insight into CSOs prior to their detection, including the self-identified drivers and deterrents of further offending, help-seeking and attempts to self-limit offending, and approaches to avoiding detection. These findings offer insight into offenders prior to their detection, and offer an alternate perspective on opportunities to detect and prevent offending.