Https://apps.nextcloud.com/apps/phonetrack (if you have a nextcloud) The nextcloud one is by far the lightest on my phone battery, the other ones would often use very much battery. But I use android and YMMV. - Source: Hacker News / over 1 year ago

You mentioned a standalone solution rather than the Nextcloud option -- just to throw it out there, here's the nextcloud plugin: https://apps.nextcloud.com/apps/phonetrack. Source: over 1 year ago

Nextcloud Maps works quite well. You can even use the tracker app: https://apps.nextcloud.com/apps/phonetrack. Source: over 1 year ago

There's a Nextcloud plug-in that does this. Hopefully that's a somewhat useful start. Source: over 1 year ago

There is PhoneTrack as an app, not OwnTracks but it does have similar features and it worked for me in the past. Source: about 2 years ago

They may be using something like Sift for security checking and something of yours was flagged. Source: almost 2 years ago

But sorry to break it to you, this has been done at a really large scale already although most consumers are not aware. One big player here is https://sift.com/ Almost every major retailer uses their service exactly for the reasons you mention. Source: almost 3 years ago

Reddit, for one. A pretty big list on their homepage. Source: almost 3 years ago