Storage type 901 is for production receipt and system create the Quant in this bin the moment you book the production receipt. Before clearing the bin my suggestion is to get a manual physical count done for the material in side the warehouse. Once yu are sure that there is a difference in between the book stock and physical stock then perform the Physical count in the system using the WM transaction (LI01N- LI04-LI11N--LI20--LI21)
As mentioned by Jurgen for interim storage type physical counting can be done provided the customizing is in place
Con of this
From system perspective you are allowed to create the Physical inventory document however it is very difficult for the user to perform the inventory as physically you don't hv the bin marked in 901 area reason being during production receipt the Production order number becomes the bin number (As per movement type setting)