Version control can be a big help. Look through closed tickets in the issue tracker, and try to find a change (bugfix or new feature) that must have, given its

Version control can be a big help. Look through closed tickets in the issue tracker, and try to find a change (bugfix or new feature) that must have, given its nature, touched the functionality you're looking for. Then try to find the changeset(s) where that ticket's change was implemented. With some luck, the changeset will include a modification to the part of the codebase you've been looking for.