Every so often, people make the entirely reasonable suggestion that if one day you see a particular person log in from locally and then a few days later they're logging in from half way around the world, perhaps you should investigate. This may work for many organizations, but unfortunately it is one of the ways in which universities are peculiar places. At universities, a lot of people travel, they do it a fair bit (and unpredictably), and they go to all sorts of strange places, where they will connect back to the university to continue doing work (for professors and graduate students, at least).

There are all sorts of causes for this travel. Professors, postdocs, and graduate students go to conferences in various locations. Professors go on sabbatical, or go visit another university for a month or two, or even go hang out at a company for a while (perhaps as a visiting researcher). Graduate students also go home to visit their family, which can put them pretty much anywhere in the world, and they can also visit places for other reasons.

(Graduate students are often strongly encouraged to keep working all the time, including on holiday visits to their family. Even professors can feel similar pressures in the modern academic environment.)

Professors, postdocs, and graduate students will not tell you all of this information ahead of time, and even if you forced them to share their travel plans, it would not necessarily be useful because they may well have no idea how they will be connecting to the Internet at their destination (and what IP address ranges that would involve). Plus, geolocation of Internet IP addresses is not particularly exact or accurate, especially if you need to do it for free.

One corollary of this is that at a university, you often can't safely do broad 'geographic' blocks of logins (or VPN connections, or whatever) from IP address ranges, because there's no guarantee that one of your people isn't going to pop up there. The more populous the geographic area, the more likely that some of your people are going to be there sooner or later.

(An additional complication is people who move elsewhere (or are elsewhere) but maintain a relationship with your part of the university, and as part of that may to visit in person every so often. These people travel too, and are even less likely to tell you their travel plans, since now you're a third party to them.)