History being hard linked to the device was one of the more surprising oddities I came across with the smart plug integration with Sense. IMO, it would be more logical for the history to remain with the device the plug was linked to at that time. I see two situations that need to be addressed:
(1) Device Not Yet Detected By Sense Connected to Plug… in this case a new device would be be created within the device list. If this device is ever removed from the plug, it would still remain in the device list. In the event that the device was removed from the plug because it had been retired within the home, an enhancement request I once saw to allow devices to be marked as “inactive” would fit in nicely here - that way the information recorded is still archived, even if these inactive devices are listed somewhere other than the main device list. If this device were ever to be reconnected to a metering plug, the record from the plug would then be pointed to that same device item within the app. If Sense ever finds this device on it’s own, the two could then be merged. The benefits here are more comprehensive device history logs and usage analysis.
(2) Plug Moved from an Existing Device to Another Device Already Detected by Sense… I suspect this is often done to identify mystery devices as you mentioned and compare total power usage vs the signature Sense credits to the device. Sense currently allows you to merge the history from a previously detected device with the plug data - this is a good thing. The issue is that this merges all the history from the previous device connected to the plug with all the history of the new device that Sense had learned. This is where the recommendation above for the history to remain with the device, rather than the plug, would add value.
I encountered this 2nd situation myself… the first device I plugged into the HS110 I was able to find was a fan that I run while I sleep. Running the fan allows us to get away with turning our thermostat up a degree or two, reducing A/C runtime. But running the fan continuously also draws energy, so I wanted to confirm the nightly cost was far less than relying solely on A/C for comfort. Once I was convinced this was the case, I moved the plug to my fridge. Sense had previously detected the refrigeration cycle, but not other fridge functions like defrost, ice making, etc. Unfortunately all the history for the fan has now been merged into my fridge history.