Season 2, in the episode entitled "I'm Okay - You Need Work" Hansen goes under cover into a drug rehab ward without authorization. In the scene showing him talking to Captain Fuller about going on vacation, Hansen's hair is a bit long on top, but short in the back. The next day, when entering the rehab, his hair is shoulder length. Over the course of a few days, it grows by several inches.

Admittedly, Hansen wears a knit cap, and it could be argued he's wearing a wig as part of a disguise. However, since he undoubtedly would have been searched when entering rehab (and again when entering isolation as seen in the episode) a wig would have been discovered.