The foundation drain sump pit is where you can expect to find radon, if anywhere.
The sealed unit you are talking about shouldn't have any place for radon to infiltrate.


Aren't the foundation sump pits perforated to allow water to enter?
If water can get in, so can radon.



In my case and the common standard here-abouts afaik
the outer edge of the foundation footing - through perforated plastic
tile - sometimes wrapped - drains into the basement sump
( plastic "pit" ) ; then is pumped out overground via tthe sump
pump - in my case a submersible model.
I suspect that any radon gas in the soil could easily find its way
into this sump ... maybe I'll get it tested sometime.
John T.