I am speaking from a totally uneducated position, but from my overly simple view on how I think Sense works for at least some devices -
When a device first turns on, it has a very unique power spike.
So the Sense starts to see a common spike. It goes “hey, that is probably a thing turning on” After some time, it notices that every time it sees that same looking spike, about 1/10 second later, there is always an increase of 400w from just before the spike happens.
After seeing that enough time, it realizes that the 400w increase just after that spike probably has to do with it, so it names that spike and following wattage “Device 1”
So it now knows what Device 1. It knows what the power on looks like and how much power to expect after that spike its familiar with, so it feel safe saying “I found a device”.
Now 20 minutes after that spike, 400w disappears from the power usage. Because it knew that the detected device used about 400w, it is probably safely assuming that the device is now off.
The problem is, many devices don’t use an exact constant current all the time, so they have to include some fuzzy logic of some sort to allow it to deal with these things.
Lets repeat the above process, but this time, it learns that after the spike, it expects 300w and then + or - 60w. It builds a model on that.
Now you’ve got a couple of these things all going at once, and several of them have an allowance for a variable wattage. Add on top devices that unknown and things start to become a mess.
The Sense has a really good signal (the spike) to when a device turns on, but then has to rely on wattage disappearing to know make a guess if something is off.
So in comes the timeouts to make sure things aren’t miscalculated. At some point in time, it must do some math and go “have I missed an off” and then times things out.
I have no idea if the timeouts vary by device type. So they know that fridges generally run for X time, so the timeout is based on the average fridge. Same for pumps. most pumps (septic, heating etc…) only run for Y time, so they set a timeout that is appropriate. for pumps.
In any case, that is my really long way of saying I think being sure about a device turning off is harder than it seems.