There is a podcast about how this works inside AIVD (civil intelligence) and MIVD (military intelligence) unfortunately it's in Dutch, but if you speak Dutch and are interested: https://open.spotify.com/show/6wZZ2ojf2XxwgfYT649wpO

They describe the process on how things can be approved and what needs to be done for that. But this thing sounds like it was more an MIVD (military) thing and I can imagine there isn't nearly as much legal due process on that side.