Finally found the "issue" with that weird truck icon on the map in Dover, the one with the yellow/orange explosive marker on top of it... It turned out to be a Balloon Winch. Never knew I put it in there... changed it to a Barrage Balloon, and there ya go... I know it's not really relevant, and not really worth mentioning here, but it had been bugging me for ages ... and I am happy it's now been fixed.
(It's not easy to find a truck, when you are looking for one, and in the mission builder you are looking at something that does not look like a truck, it looks like a Barrage Balloon... amongst many others ... )