If Arc is picking a place that’s far away from your real location, that means there are some visits assigned to that place incorrectly, making the place appear to have an incorrect centre location and overly large place radius.
Tap into the details view for one of the visits and have a look at the orange circle on the map. That circle will be overly large and incorrectly centred.
To fix the problem, tap through to “Place Details” at the bottom of the visit details view, then “Total visits”. Scroll down that list until you find the visits that have been incorrectly assigned, then correct them. That’ll solve the problem!
This is already built into iOS. iPhones use a combination of cell tower triangulation, wifi hotspot triangulation, and GPS/GNSS triangulation to determine your location. In built up city areas your phone is often most heavily relying on wifi hotspot triangulation rather than GPS/GNSS.
Yep! iPhone apps are not guaranteed to be allowed to stay alive and running in the background. iOS can terminate them at any time for any reason, and that does occasionally happen. When that happens you’ll end up with a data gap in Arc until the app is restarted. iOS might restart Arc automatically, but that might be delayed by tens of minutes or even hours or days. So if you care about having a complete timeline without gaps, it’s something you want to avoid.
Arc Mini is there as a fallback, to take over recording if Arc App gets terminated. The two apps communicate in the background, keeping track of whether the other one is still alive or not. If the one in charge of recording disappears, the other takes over, thus avoiding data gaps.
Because only one of them will be the active recorder at a time, with the other in a sleeping/standby mode, only one of them will be consuming energy for recording purposes. So there’s little to no extra energy cost from having them both running.
Arc Mini also takes on some of the job of doing daily housekeeping tasks, sharing that load with Arc App too. There’s a bunch of things that need to be done overnight / when your phone is idle, to keep the database in order, activity type models up to date, data summaries built, etc. Both Arc App and Arc Mini schedule those tasks with iOS, then iOS might start the task on one app or the other, so in practice those jobs are shared between the two.
For that 10% of battery use, be wary of what you read in the Settings → Battery view. It’s not quite what it seems, and often misleading. It’s not just showing you energy used while on battery - it’s also showing energy used while plugged in to power. So a good part of that 10% may well have been some of those background housekeeping tasks that were performed while the phone was plugged in to power.
In practice, both Arc App and Arc Mini use extremely little energy each day. As an example, my ageing iPhone 11 Pro with an almost dead battery, that I use as a secondary test device, continues to record without problem for at least 24 hours on a single charge.
Arc App / Arc Mini’s energy use while recording (or in sleep mode while stationary) is so low that it barely makes an impact. The overwhelming majority of energy use happens when the app is in the foreground, building and updating the UI and map. During that time the energy used by the phone’s screen is also credited to the app. So for example if you left a completely idle app open in the foreground all day, with screen lock turned off, that app would end up being credited with near to 100% of energy use, even though the app wasn’t doing anything other than sitting there idle.
So yeah, that Settings → Battery view appears simple and obvious, but most of the time it doesn’t mean what we think it means 
