Here's an odd issue I encountered yesterday. The 172 just finished with both the annual and the Altimeter and Static check, completed by different shops. I have a fairly extensive post MX flight process that I go through to check everything out. I typically do an alternator check on runup but since the plane had sat around for a month at two shops and was a little sluggish on start I decided to wait and do that in flight. Once in the air I ran through all my little items I do to ensure everything is working well. One of the last things I do is fly a full RNAV approach to validate the GPS and CDIs are working well. Decided that would be a good time to mess with the alternator again so I turned it off... however it didn't look like it was turning off as the voltage remained fairly constant never dropping below 14. After a few minutes I decided to turn the master off... well nothing turned off, as if the alternator was still powering everything, about all that changed is I could hear some type of repetitive noise on the headphones. Voltage didn't change even with the master (and alternator) off. I cycled through this a few times with no changes, left it on and did the approach. In short master works like normal with the engine off, but once it's turning you can't turn the master off.. I validated this on the ground after I got gas. The A&P is going to look at it but was curious if anyone has every ran into something like this? Could it be something as simple as a messed up alternator switch where it's not actually turning off? I thought it was mechanical so not sure how that would work. Or a misplaced Blead on the contactor? Thoughts?