Sub Dots?

[Blathergut]

Subs (maybe others?) sometimes have a bluish dot in the lower right-hand corner. What is this?

[Peter Stauffenberg]

That dot means the sub is hidden, i. e. can't be spotted by the enemy. If a sub has attacked then the blue dot will disappear. It will also disappear if an enemy naval unit suddenly sails through the hex the sub was hiding in.