They were indeed standard equipment on all North American models, so it was likely misplaced by a previous owner. It's not a 'dongle' and even though it does have a ring on top, it's actually too large to keep on a key ring (it's about 4 inches by 1 1/4 by 3/4's) so most owners keep them either in the glove box or in the house, which is where you would likely use it the most. It doesn't have anything to do with the seat heater, but you would need to have it to pre-heat (or pre-cool) the car, if that's something you think you would actually to do - I used one of mine for that a few times, but found it more of a pain than it was worth. It can be used for delay charging, but it's design for doing that leaves lots to be desired, IMO - I've never used either of mine for that purpose
The bad news - Replacement cost is about US $1K!
Worse news, you can't buy a used one and use it with your car - It must be programmed to the car and I'm not sure the dealer could re-program a used one to work with your car. The good news . . . . not having the remote is not really that big of a deal, IMO. You would certainly miss having one if you needed to regularly pre-heat the car while still hooked to the EVSE on cold mornings, but other than that, they're not of that
much use. I have two iMiEV's and I have the remotes for both of them laying around here somewhere, but I've not used either of them for at least a couple years, probably longer
Probably the best news about looking at a used car with a missing remote is . . . . you can argue to lower the price when you tell the dealer that 'it's really
important' (never hurts to fib a little bit) and replacing it is going to cost you, the buyer $1K, so how about we split that cost and you lower the price of the car by $500?? That works . . . . sometimes