Lets see, he just brutally killed a judge, and several other people in the area. Why would he keep something as trivial as using drugs a secret after murdering people? There is no honor among thieves. No doubt he ratted her out on his first interview. Let me guess, you think this was an honorable guy right? Your a defense attorney. Do you think they searched that apartment? Well if you don't know, I'll tell you. After he was in custody, they searched every square inch of that place, looking for that relationship/connection and you can bet they found at bare minimum, traces of meth. I not only believe the suspect told them, I believe she did too when confronted.
And if it didn't happen the way I just described, they weren't doing their job. I am telling you, when they went in to that apartment, they found at minimum, traces of meth or obvious evidence of use. The gig was up almost immediately as far as the drug issue. Otherwise, they just didn't do their job.