A new ARRIS device connected to my wifi yesterday

AFAIK, that's an Arris X1 / Xi6-A wireless cable T.V. box. Do you have one at your premises ?

No, I have only a Comcast-provided XG2v2-P.  I tried to respond to your post, EG, but found that it did not post.  I'm sending this to confirm that we have a back and forth established.  Thank you.

Possibly a neighbor's box found yours first and is connecting to it. Not sure if an Xi box must connect to your own gateway or not to work properly and get the correct channels.

Thanks for the suggestion.  Were a neighbor's Xi box to try to connect to my network, it seems it would need the wifi password(s) to be successful as both the 2.4 and 5.0 GHz radios are password protected. Perhaps I am misunderstanding how devices can join.

If you have a Comcast gateway, they have special SSID's used for Xi cable boxes and devices like security system sensors. They are separate from your home network.

Very interesting!  The device info on the Xi showed a subnet IP assignment of 10.0.0.254, which I would assume to be outside the dynamic assignment range (highest being 10.0.0.253) and a host name/MAC identical to the MAC's of the 2.4 and 5 Ghz radios in my Comcast provided modem/router, but with the last digit on the right elevated from 3 and 4, to 7.  My response was to "Pause" the interloper "Until I un-Pause".  Are there any other preventative measures a Comcast customer can/should take to be less generous in sharing their bandwidth?  Thanks again for your insights!

The exact same thing just happened to me. What is this? My network is secure and password protected. 

Exact same thing just happened to me too!