Hey there, thought I might be able to provide some additional context. OP, the reason people are calling out your brothers app as BS is because of how location works on iPhone.

There are two ways location services can work. 1. If you are checking location via an online web app, then they can detect location via IP. This can be spoofed via a simple VPN, and is how things like unlocking Netflix in a foreign country works.

The second way location services work is by using the GPS in the phone. For security reasons, this is handled by IOS itself and while apps can request to read the location, they do not have the ability to set the location. Apps on the phone itself use this method. At best your brother can turn off location services, but he cannot “spoof” the location that shows up in find my iPhone.

The one case where this changes is if the iPhone is jailbroken. iPhones that are jail broken have “no limits” so to speak and anything can be modified, including the location data.

Another way to spoof location data is by taking advantage of iOS’s modular nature to connect it to an external GPS receiver. I don’t know a lot about these devices or how they work. I know pilots use them to add reliable GPS to their devices since the internal gps isn’t rated for flight. It’s possible the application you mentioned is emulating one of these devices and purposely feeding the phone with bad data, but I’m not sure.

But to answer you question, you can’t really know. You can track your brothers location over time to try and identify and sudden moves or unexplained jumps, but that’s it. If the system didn’t think the system was real it wouldn’t report it to begin with.