To slip through the net means to go unnoticed or undealt with, and in this context the speaker is saying that they will not let another spook go unnoticed.
Context: Dee, as we are now on first name terms, I'm not letting another spook get away unnoticed or undealt with. 
Hypothesis: Dee, as we are now obviously on first name terms, I'm not letting another spook slip through the net.

Is the hypothesis incorrect? (Yes, no, or unknown)?