Easy answer: No.
More in-depth answer: It sort of depends.
If you DON'T connect to the plant's WIFI, then no, they cannot "see" or track your phone, let alone be able to match your phone to you and watch your location as you walk around the plant. If you're just using the cellular connection, then the company can't see anything - that traffic doesn't go through any of their equipment.
Now if you DO use your phone to connect to your plant's WIFI network, then they MIGHT be able to track you. Depends on what they require you to do to your phone. But for the most part, it would be the same as using a company computer to access the internet. It's safe to assume that the company CAN track your usage, even if they don't.
Would this be legal? Yes - assuming they disclosed up front that using the company WIFI meant the company would be able to track and monitor your network usage, or even your location. Bottom line - their network, their rules.
Now if you talk to most IT folks, they'll tell you that they really couldn't care less what you do so long as it doesn't damage the company in some way. They'd like to believe that you're a responsible adult, and as such, knows to use their network in a responsible manner. At the same time, their job is to protect the company's security and HR policies. So while they may have the tools to see everything you do online through their network, they'd much rather not have to do that.