TL/DR: The phone shows an additional hotspot connection if it is connected to my laptop via cable. Changing the HotSpot password on the phone will cut access to connected devices but it may take several minutes for the device (my laptop) to register the disconnection.
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Here's what I experienced, from the top.
I was working on my laptop, using my iPhone 6's hotspot for Internet access, when I glanced at my iPhone and noticed that the blue bar that appears at the top of the screen when a device is connected said 'Personal Hotspot: 2 connections'.
I thought this was odd because my iPhone and iPad were the only two devices in the vicinity that have ever accessed my Hotspot and my iPad had been on airplane mode for days. I checked my iPad and confirmed that is was still on airplane mode, that all wireless connectivity was off, and that it was not connected to my hotspot. (For clarity - I'm far from home at the moment and have never given Hotspot access to anyone else.)
I was a little worried because I've got a finite amount of data from my mobile provider (and big overage charges) so I wanted to terminate any unknown connections as quickly as possible. That's when I changed the HotSpot password on my phone and cycled the HotSpot connection on and off. The '2 connections' reappeared when the HotSpot was turned on, even though the new password had not been entered on my computer. I cycled the wi-fi on my computer as well and the '2 connections' would switch to '1 connection' when the laptops wifi was off but the iPhone was still plugged into the laptop (via the USB/lightning cable).
Initially this looked to me like 1 known but uncontrolled connection (the laptop wifi connection that wasn't updating the password credentials immediately) and one unknown connection (the one that remained when I turned off the laptop's wifi).
After a few more minutes of playing around (as mentioned in my previous post) I discovered that this second connection only appeared when the phone was connected to the laptop via cable. Just checked again (6 hours later) and it's still showing this second connection when connected by cable.
I just reset the password on the phone again (while the '2 connections' were active). The '2 connections' flashed '1 connection' briefly and then returned to '2 connections'. My computer is no longer connected. After 1 minute my iPhone shows '1 connection'.
What confused me last night was the '2 connections' and the fact that my computer's wifi didn't get cut off immediately when I changed the Hotspot password. (I just tested this again and found that although the Hotspot connection icon on the laptop continues to be shown (the two interconnected links) there is actually no connection.)
Anyway, I'm no longer worried about unauthorized connections now that I know the second connection is from the cable. I'll continue to look forward to some kind of app that shows which devices are connected and I'm curious if anyone else can confirm this.
Thanks.
15" Macbook pro, OSX 10.9.5
iPhone 6, 8.1