This device was initially labeled water heater but my water heater is gas. This device comes on every 30 minutes or so and uses roughly 1500W for 2 minutes … any thoughts to what it might be?
Do you have a Bunn or Keurig coffee pot that keeps water heated up all the time?
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I do have a cuisinart coffee maker. I had not thought of that. I have just unplugged it and will let you know if that was it.
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Well it wasnt the coffee maker, seemed like a good candidate though. And I don’t have an instant hot water heater so I need more suggestions 
This looks a lot like our little electric space heater that we use to warm up the master bath. Similar power and interval, although ours does vary depending on how cold it is outside.
Could also be a hot tub
I’ve gone thru the house a couple of times now and all of our space heaters are unplugged and I do not have a hot tub. ???
Some mini-split or ductless heat pumps have auxiliary heat strips installed
My heat is gas powered so no heat pump.
One more thought. Do you have a large forced air furnace that is also set to run the fan a minimum amount of time for each hour ? The reason I ask is that two of the spikes one the left (#s 2 and 3) are kind of shaped like my heating cycle with the igniter spike first followed by a larger blower spike. Usually you would see current/power overshoot from the motor load, like in my furnace screenshot below, but perhaps your motor has a compensation capacitor.
Fan cycle on left, heating cycle on the right.
My heat is hot water heat (gas) which has a electric motor that pumps the water thru the pipes. I removed power supply from the pump and the mystery device is still powering up. 
I think I should be able to detect which main it is using, and if so it seems like I could then eliminate half the circuits, does this seem reasonable?
Do you have a heat pump? Some of those run heaters on a time clock to keep the workings warm in cold climates
no heat pump.
I may have misspoken about the heat pump. I do have an A/C unit (which is maybe a heat pump?) . Which maybe it kicks on a heater to keep the workings warm. I may try throwing the circuit breaker to the A/C to see if that is the culprit.
Can you post just the device usage?
Further testing leads me to believe the mystery device might be the sense device itself.
As you can see from the image below, the mystery device turns on at 11:40 and off at 11:42. While it is on there are 3343 watts being used (1500 watts of which is the mystery device), 1282 on the left main and 2061 on the right main.
Seconds after the mystery device turns off you can see the total watts have dropped roughly 1500 watts and that appears to be shared across both the left and right mains.
The fact that the drop in wattage is shared across the two mains, does this imply that the mystery device must be a 220 device???
If so, then it must be the sense device because all other 220 circuit breaker were turned off.
Well that certainly deepens the mystery, but Sense only consumes around 4W.
Am I correct in thinking that since the usage was spread across both mains, that it must be a 220 device ??
Not a chance it’s Sense. That draws less than 5 watts and 1500 watts would vaporize a little piece of electronics like Sense immediately.
Having determined its a 220v device eliminates 98% of the devices in a home. There are only a few choices, large heaters (stove, oven, hot water, hvac, etc), large motors (although this signature doesn’t seem motorish) , etc. Things like refrigerators, freezers, space heaters, etc would all be 110v.
Check your breaker panel for 220v breakers for clues. It also looks like something on a timer.
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