Two connected devices

I've been using this setup, of an Android phone as a hotspot, with a
laptop running win7, for about 5 months over the last 13 months, and it
works fine.

Today, under Portable Hotspot, it shows that I have two devices
connected. One is my PC and the other is a0:28:ed:ad:4e:4e

Because I spent an hour trying to install bluetooth drivers yesterday
and almost finished, I keep thinking the second connection is bluetooth,
but for Bluetooth, there is no listed connection.

https://aruljohn.com/mac/A028EDAD4E4E shows the first 3 sections go with
vendor HMD Global Oy !

"This database was last updated on Tue, 19 March 2019"

For Lookup Vendor, it shows not found.

https://www.whatsmyip.org/mac-address-lookup/ shows the same thing.

Hmd Global Oy, branded as HMD, is a Finnish mobile phone company, made
up of the mobile phone business that Nokia had sold to Microsoft in
2014, then bought back in 2016. HMD Oy began marketing smartphones and
feature phones under the Nokia brand on 1 December 2016.

So is someone else's mobile phone using my mobile phone as a hotspot?
The apartment has no wifi, and I have two roommates, neither of whom are
here now, there is an apartment upstairs**, and there are some workmen
just outside today.

**I don't know the people upstairs but when I noticed this between 10
and 11AM, they were probably at work, not here.

I have more data than I can use each month, so is this a problem?

I don't remember if there was a way to use encryption or require a
password, but wouldn't a second hotspot user just be getting data to and
from the cellular connection to the web, not from me?

(Osakeyhtiö ("stock company"), often abbreviated to Oy, is the Finnish
name for a limited company (e.g., Ltd, LLC, Inc.). The Swedish name is
Aktiebolag often abbreviated to Ab. )