Have you checked your vent boots?
Here's a common sight when I'm walking a roof inspecting. Roof boots with cracked seals. Although your shingles may be rated for 25+ years, the roof boots tend to last far shorter. The ones in these pictures are on a 16 year old house and have probably already been cracked for years. I've seen them beginning to split as early as 7 years depending on sun exposure. Luckily, having these boots replaced is generally inexpensive. Next time you have your gutters cleaned, consider asking the contractor to take a quick picture of your vent boots. It may be time to consider replacing them and if your thinking about selling your home, consider getting a seller's inspection so that minor details like this don't hinder a sale. It is always cheaper and easier to repair or replace deficiencies when you don't have the ticking clock of a contract hanging over you. (Standard disclaimer: these blogs are not intended to be DIY instructions. Please do not undertake any repairs that you are not qualified for and comfortable doing)