Chest freezer found
I assumed it was actually the chest freezer since it is the last undetected device in the house I can think of similar to a fridge. I rarely catch the chest freezer running, so it makes sense that it would take Sense longer to fully learn its routine when it cycles less than other appliances.
To confirm it is indeed the chest freezer, I enabled the ON/OFF notifications for it and waited for the notification. Sure enough, when I received a notification this afternoon, I checked it and it was running.
I went ahead and selected "Freezer" from the community names, which was listed as 9% that other Sense users named similar devices.
Now I have another little bubble to look at and hopefully an ever shrinking "Other" bubble.