My helmet, a full coverage type, when left with the bike is secured with a cable lock. I sometimes carry a second helmet, especially when using the bike with the sidecar or expecting a passenger. But I usually keep the helmet that I am using, and the riding jacket with me when I leave the bike. and an alarm lock on the bike if I am any distance away and out of sight of the bike.
But if, for some way or other the helmet was stolen from the bike, I would telephone and report it to the police, and indicate that I was going to have to ride to get to a place where I could buy a replacement helmet. Then, if on the way to buy a helmet I was stopped by the police for not wearing a helmet, I would explain the situation and ask that the officer contact the police message centre for confirmation. If charged for "no helmet" I would get all the officer's details and when (and if) the case came to court I would have the information on the date and time of theft, the date and time when stopped and charged and the date, time of sale and bill for the new helmet - and the new helmet - to present to the judge. I would bet that the judge would look at the officer and say, "Case dismissed!"
AFJ