It's possible your tire has gotten thinner and allowed a tiny puncture to your tube. Or there might not be anything at all wrong with the tire and something still worked it's way in enough to poke a small hole in the tube. It happens.

When you get the tire off you can also check the rim tape that covers the spoke holes to make sure it's in good shape. If the tape is worn around a spoke hole, sometimes that will wear a small hole into the tube as well.

I hate to throw anything away unless it's completely shot so I patch tubes. In fact, I had my first flat in about 2 years on the way to work this morning. The spare tube I put on already had at least two patches on it. Now the tube I just took out will get one.

LBS mechanics don't patch because a tube costs them less than what it would cost the mechanic to patch a tube. Plus they can probably charge for both the tube and the labor.

Nobody pays me to replace a tube so I can justify the time it takes to patch one. Besides, we generate enough garbage as it is.