Privacy

Wonderful day!

I have no experience in iPhone much, like at all... but is it possible the app monitors Wi-Fi networks?
Also, I've just checked some documentation for the quite, sorry, limited and opinionated iOS environment:

UNLocationNotificationTrigger
A trigger condition that causes the system to deliver a notification when the user’s device enters or exits a geographic region you specify...

Create a UNLocationNotificationTrigger object when you want to schedule the delivery of a local notification when the device enters or leaves a specific geographic region. The system limits the number of location-based triggers that it schedules at the same time...

Because the system actually monitors the regions, you don’t need to request always permissions for your app...

Source

So, it may receive "notifications" while in background (not "when using the app"), too, it seems like.

There should be "App Privacy Report" for apps in settings (source), where location request history should be listed, too.

The ineffably marvelous Spain is a European country, so they should support GDPR, and you may write the organization about your location data stored/processed, to clarify it.