I think the source is the "former official". I'd imagine that most of the time when allies are caught spying on the U.S. the public doesn't get to know about it

I think the source is the "former official". I'd imagine that most of the time when allies are caught spying on the U.S. the public doesn't get to know about it, so it might not be possible to source that claim without talking to someone with inside knowledge of this stuff.