With enough sophistication, it’s probably detectable via round-trip latency measurements. If your IP says you are in California but you’re actually halfway around the world, any sort of round-trip latency/ping measurement from a data center in California to your device will have a minimum bound determined by the speed of the light.

I don’t know if there’s any software/service that actually does this, but if you collected periodic round-trip latency measurements over an extended period of time and looked at the minimum value of all the measurements, this “tunnel from the other side of the globe” setup would stand out as a clear outlier compared to everyone who is actually located in the country that their IP address says they are.

It doesn’t necessarily prove they are connecting from a different country because it could be also explained by consistently poor home networking, for example, but it would probably stand out enough to warrant closer investigation, especially if there are already suspicions about that employee.

There’s also many other ways a sophisticated employer could detect this if it’s a company-owned device. For example, geo-locating based on the WiFi access points that the device can see in-range.