According to Deborah Madden's statement to police which ultimately led to her forced resignation and the great shaming of the SFPD, she was only doing what everyone else did, swiping cocaine that was kept as evidence and never worrying much because everyone "laughed" at whatever weight discrepancies came up along the way.
Debbie insists she only ever took home "trace cocaine" -- the stuff that was left over on surfaces which your cokiest friends usually like to lick up and rub on their teeth.
More sad facts: she only took home lab vials in order to mix paint for the plastic models she builds. And she only started doing coke a few months before, after getting some a friend, because she thought it would help control her drinking. "I don't think [it] worked," she said.
So after being around it in the crime lab for three decades, she only became a coke fiend and started stealing the stuff in her twilight years, because she was late in finding out that it kind of sobers you up.