Modules transferred to Depot disappear.

Trevzor: Heyo!   The reason you're top modules are not in your depot after you sell the tank is because they were still mounted on the tank when you sold it.   Selling a tank in game is a lot like selling a car to a dealership. You can't sell the dealer a car that is not a full car. Same goes for tanks. Whatever is mounted to the tank at the point you wish to sell it gets sold along with the tank. The modules that are marked as "Send to Depot" are all of the modules that where related to that tank but not mounted on it.   To make sure that your modules are transferred to the depot, definitely mount stock equipment on your tank before you sell it (I found this out the hard way when I spent all of my credits buying my 212 and not having the top cannon for it despite it is the same for both artillery )