The ICE is programmed to turn on once a month to pressurize the fuel system and perform system checks. This is normal. If it remains on after a couple of soft resets, that's abnormal and might indicate a leak in the fuel system or a software problem or something else.
It is my understanding that the Fisker engine will turn on every once in a while to make sure the Gas does not go stale. I have been told that gas stored above 70 degrees Fahrenheit lasts at most only 3 to 6 months and below 70 degrees is 6 month to a year.
@Joeker, As already mentioned, the Karma's ICE does turn on for a number of legitimate reasons, such as when the fuel filler door (not the gas cap) is opened, and to re pressurize the fuel system or to just exercise the ICE if you have not run it for a while. There is also a bug in the software that creates a weird stutter effect sometimes, discussed in this thread.
When the ICE does come on, it has to run long enough for the emissions system to come up to temperature before shutting off again.
So if this is a one-time event, it's probably nothing to worry about. If it is happening a lot, however, you may need to make sure that the fuel filler door sensor is working correctly and that your fuel system is not losing pressure. In my car, for example, if I go more than 2 days on EV-only mode, the ICE turns on for a few minutes on my next drive. Since I use the ICE pretty much every day, it's not really an issue for me, but I will probably have the fuel system checked for pressure leaks at the next service interval.