Hi @mucip. If your code is iterating through an assembly's BOM, then not only is the assembly open, but all the files that the assembly is actively referencing have also been 'initiated'. What that means is, they have been partially loaded into Inventor's session memory, but not fully or visibly opened yet, so no document tabs for them. If you have Inventor's 'status bar' showing, then you will see two numbers showing within its right side. One (furthest right) is the number of open documents in session, while the other is the total number of occurrences in the active document.
With that in mind, there is a property of all Document type objects called ReferencingDocuments that may help. It will only be able to find/include the other Documents that are currently loaded in Inventor's memory, and are currently referencing that Document. So, if the drawing for that model document was already open, or somehow invisibly loaded into Inventor's memory, then from that model document, you may be able to find that drawing within that collection. However, the assembly will also be referencing that model file, and any other sub assemblies that it is in will be in there also. So, if it is in there, it may not be the only one in there. Plus, if there are any drawings open for the whole/main assembly, then that drawing will also be referencing that model document, because it is referencing the assembly, and it is referenced by the assembly. But if the drawing is not already open, then this property would not be able to help any. Maybe if you had something like Vault installed and were using that, and all these files were currently being 'managed' by it, then there may be a way to use Vault to find the drawing file that references that model file, because it keeps all file references in an active database.
The Document.FindWhereUsed method works about the same way. It will only search within the other documents that are currently loaded in the active session of Inventor, and won't search your entire file system. There is a standalone tool that gets installed with Inventor named "Design Assistant", which has some functionality for 'Where Used', and that will actually search throughout your entire project space, of file system, if you want it to, but if that area contains tons of files (like mind), then may take a really long time to do.

I do not know of any other property on the model file side that will point to any drawing that may have a reference to it. If that were the case, then every time you create a drawing, and add that part, or an assembly containing that part into it as a view, it would have to 'modify' that model file, to add something into it about the reference to it that was just made. Managing those associations in both directions, to support functionality like 'Where Used' is large part of what Vault does for us. I do not yet have Vault, but I believe we will be getting it soon.
Wesley Crihfield

(Not an Autodesk Employee)