If our corporate people (I'm one of them) want to visit one of our sites, we have to sign in, no ifs or buts. If it's a site we regularly visit, we have to undergo a site induction and the level of the induction is based on is it office or manufacturing facility or both that you will be visiting. Our sign in systems use a tablet, with sign in software where you can choose employee or visitor, contractor etc, then enter your name or phone number, it checks if you're inducted, if not there is an induction on the tablet which you have to complete, the system issues you a pass (printed label with your picture), or in my case, I'm issued with a pass, and that pass is electronic and you can only go where it allows you to. When you leave you sign out. I've seen systems where if you forget to sign out you get a test message to see if you are still on site or not, if not you can sign out via a link. Simply, no one can get onto our sites nowadays without signing in and meeting up with who they need to see.
As noted above, it's imperative that it doesn't matter who is visiting your site, corporate or not, if they are properly inducted, wear a pass, must be escorted around the site if not inducted. It's a safety issue, you have a site emergency, how do you know who's on site or not, what PPE do they need (if any), it's a food defence issue, what are they doing on site, who are they with? It's a GFSI requirement - they are not being compliant by not following a defined visitor policy, and they are a site visitor, corporate or not. This really needs a big push to ensure compliance - there is too much risk to the site, the visitor and the company if they find it "too hard" to sign in.