Mentoring is traditionally viewed as a dyadic, top-down apprenticeship. This perspective, however, overlooks other forms of informal mentoring taking place in everyday activities in which developers invest time and effort, but remain unacknowledged. Here, we investigate the different flavors of mentoring in Open Source Software (OSS) to define and identify implicit mentoring. We first define implicit mentoring--situations where contributors guide others through instructions and suggestions embedded in everyday (OSS) activities--through formative interviews with OSS contributors, a literature review, and member-checking. Next, through an empirical investigation of Pull Requests (PRs) in 37 Apache Projects, we build a classifier to extract implicit mentoring and characterize it through the dual lenses of experience and gender. Our analysis of 107,895 PRs shows that implicit mentoring occurs (27.41% of all PRs include implicit mentoring) and it does not follow the traditional dyadic, top-down apprenticeship model. When considering the gender of mentor-mentee pairs, we found pervasive homophily--a preference to mentor those who are of the same gender--in 93.81% cases. In the cross-gender mentoring instances, women were more likely to mentor men.