They might not wear them out on patrol or manoeuvres, but back at their tents/barracks, I would assume some if not all have their personal phones. You only need a couple to track them.
I also read once Strave/Fitbit type trackers was rife at army bases and used to work out patrol routes.
"NRKbeta is NRKs sandbox for technology and media. We write about media, the internet and new technology with a focus on you as the user, and what we at NRK do in this field. We call it a sandbox because we want to test things out, be curious and find out how things change. And bring you, the users, with us on this journey."
EDIT:
I also think it's important to contextualize this journalism with the current debate around the Norwegian contact tracing application.
The application has been heavily criticized for the collection of GPS data for research usage and track behaviour when new guidelines are announced. They claim this data is going to be "anonymized", but alter clarified it would only be "pseudonomized".
It is also unclear if the data collected is going to be deleted in December, when the app is set for deletion by the current regulation from Stortinget.
Now, other uses might require more time. If you really need to see where the person has infected others and this is your tool, it might not be enough time. It is too early to tell, though, and I'm not sure how well phone inspections would go here in Norway nor how many people would download the app. It would make me more likely to leave my phone at home if, you know, I had much life outside of home.
Back when Wikileaks released the Afghan War Diary, I wonder what would have happened if rather than a whistleblowers we would have people buying data collected from soldiers smartphones in order to reconstruct the material. It should be pretty easy to identify colaborators by which smartphone gets into contact with someones else smartphone thus reconstruct who is working with who.
The NRK subsidiary NRKbeta has "connected the dots" from that data set. In this article they present how they could track down military personnel visiting restricted military sites in Norway, including the disputed radar installation in Vardø, close to the Russian border.