I've been encountering similar issues with my early 2011 Pro 13". Over several months the situation degraded to the point were the wifi icon would show "no hardware installed", or just be blank (empty), or show full bars but actually not be connected. In network preferences, it wouldn't show any networks, and would not turn on. A big frustrating mess. Most recently needed to be hard-lined to the router to operate.
Made an appt. with the Genius bar. They got rid of malware, did the energy reboot (forget what it's called). Didn't really do much. Follow-up phone call with tech support ended up having me re-install El Capitan (several months ago had upgraded from Snow Leopard due to outdated browser).
In the end, nothing worked. I had read this thread (among many others), and was ready to possibly replace the wi-fi ribbon cable and/or wi-fi card. Decided to attempt the "fix" promoted by feeel (Phil).
Had never removed the bottom panel before. Went to ifixit for guidance. It was kind of dirty inside, so disconnected the battery, and gently vaccumed. Then did the folded paper method posted by feeel. Attached the battery, screwed down the bottom panel, and powered it back up.
Two my pleasant surprise, wifi activated and found the right network. After a few hours, I left that location and returned home. It is still active, and connected to my home router.
So, this is just day one, but so far it is behaving better than the past several weeks.
Thanks to all contributors, esp. (for now!) Phil.