Tom, check https://www.askmehelpdesk.com/heating-air-conditioning/leak-near-radon-vent-basement-51393.html#post240346

I figure you are the best to track down where a leak is coming from.

On it Lab, Here's my answer to him.

What's your location and the weather temp at your home?
Define "moderate rain" by inches in a 24 hour period. Your pictures aren't titled. Topmost is the basement where the leak was noticed? Next the picture is a attic shot with the radon vent on the left hand side? The picture under that depicts the vents exiting the attic to the roof with the radon vent on the right hand side and the bottom picture shows me, what?? Two vents that have vent caps and two in the background that are not. I don't like to be forced to guess at what I'm seeing so if you don't mind please comfirm that my observations are correct. I'm with Labman on this. It's doubtful that you have a internal leak caused by rain water inside the radon vent. If I had to track it down I would start by taking a hose up on the roof running water down the radon vent before anything else got wet. Did it leaki? No? Then I would load the area around ALL of your vents with water and check some more for a leak. Still nothing? Now we get back to why I questioned the rain and temperature. I'm thinking condensate. Where warm humid air from the soil meets the dry cold of the attic. These are just off the top of my head but you sounded like you needed a place to start. Regards, Tom