Any penetration point in your roof is considered a leak prone area and should be the first place you look when trying to locate a leak. Pipe vents are the most common type of penetration found on a roof and are from bathrooms, dryers, and some high efficient HVAC units. Each PVC pipe is typically wrapped in a boot and rubber doughnut. However, these boots can leak over the years.
Hot summers can eventually crack and tear the rubber seal around your vent pipe. These should be inspected every year, to keep you from finding out, after the fact, that you have a roof leak. In addition, when the rubber separates, water will enter your attic space and typically track down a rafter. This allows the water to enter your ceiling from a place far away from where the actual leak took place. Just because the leak is not showing up in your home directly below the vent pipe, inspect anyway, this may solve your problem.