What would cause me to occasionally hear water running down my exterior drainpipe when absolutely no is present and one else lives in house?

There are a few possible reasons why you might hear water running down your exterior drainpipe when no one is home and no water is running inside your house:

- Condensation: Condensation is the process by which warm air cools and turns into water. When the air in your home is warm and humid, it can condense on the cooler surfaces of your exterior walls, including your drainpipe. This can create the sound of water running even when no water is actually present. Condensation is most likely to occur in humid weather, especially during the summer months.

- A leak in your plumbing system: A leak in your plumbing system can also cause water to run down your exterior drainpipe. If you have a leak, the water may be running continuously, even when you're not home. Check your plumbing system for any signs of leaks, such as dripping faucets, wet floors, or water stains.

- Overflowing gutters: If your gutters are overflowing, they may be draining water onto the ground near your drainpipe. This can create the sound of water running even when no water is actually coming from your drainpipe. Check your gutters to make sure they are clear and free of debris, and that they are draining properly.

- A problem with your drainpipe: If your drainpipe is damaged or blocked, it can cause water to back up and overflow. This can create the sound of water running even when no water is actually coming from your drainpipe. Check your drainpipe for any damage or blockages, such as tree roots or debris.

If you are concerned about the sound of water running down your exterior drainpipe, it's best to call a plumber to take a look. They can help you determine the cause of the problem and recommend the best solution.