Depending on the amount of fumes visible (it’s not actually steam) this could be a small or large problem. Here’s how that works: A small amount of white fumes from the oil filler cap is not so bad, but what you don’t want to see are gales of the stuff blowing out when you remove the cap.

These fumes are caused by pressure from the combustion process in the cylinders finding their way into the crankcase. As an engine wears, more and more pressure can find its way into the crankcase, increasing the fumes. When the fumes increase, this is a clue that your piston rings (among other things) are becoming worn over time and kilometres.

But you could also be looking at a car with a faulty PCV valve. This valve has the job of removing the fumes and sending them back through the engine to be burnt a second time. But if the valve is faulty, the engine won’t be vented sufficiently and the fumes suddenly become visible again. So, have the car checked out at a workshop who will be able to perform a compression test to tell whether the engine is in good internal condition or not, as well as check the PCV valve for correct operation, If the problem is the latter, it’s an easy, cheap fix.

They wouldn't have worked on it in the car park, I'm sure of that. I would take it up with the dealer and have them explain why you car hasn't apparently been driven after being parked in the lot.

You need to take this problem directly to Hyundai and request the company’s help to fix the problem. It’s possibly an electronic problem that affects the engine when cold, it could be that it’s not getting the fuel enrichment that it needs to start cold. The company is the one that can fix it for you and there’s no reason they can’t.