Since I have just been "reading" (audiobook really) White Night I recall something about this there.
When Dresden thinks Helen Becket is part of the killings Abbey the precog talks about mobile phones and how with her modest power hers works most of the time and Dresden comments that if Helen has a mobile phone she at leased wasn't hiding a bigger talent.
I would guess to the white council bit that since player characters are probably going to be quite powerful and so will have pressure to join and the simple act of refusing will immediately put them under a lot of suspicion. I mean they are the white council who doesn't want to join, clearly it is because they are hiding something they don't want the council to know therefor they must be a warlock.