Obsolete. Referencing in AC10 and later doesn't work this way.
It sure is nice the way detail markers know where their drawings are placed. It sure would be nicer if we could as easily refer to any placed drawing. If you could place dynamic drawing and sheet references in section marker objects, and call out enlarged plans, and write notes of the form, "Do this like this, see detail A3-5/12." And since you can't place details in detail windows, use an object detail marker there.
Well, you can refer to any placed drawing. But I wouldn't call it easy. In fact, I think this is the most annoying worthwhile thing in all of ArchICAD.
Here's how. We'll do the drawing number first.
Import the drawing to a layout.
Right click on the drawing (in the layout or in the tree) and choose "Set as Autotext Reference".
Activate the text tool. Start a text block by double-clicking. In the text editing area (at the cursor), right-click and choose "Insert Autotext for [DrawingName]" -> Drawing Number. This results in a blob of gibberish like "< DRAWINGNUMBER_R ><9BEA5D4E-8700-11D8-8AA9-000A95A7B33A>". Copy this. Tip: Cmd+A works to select all while editing text. So, Cmd+A, Cmd+C.
Return to Archicad. Open whatever it is that needs the reference, e.g., a detail marker object or a text block. Paste into the appropriate field or location.
Of course, you usually need the layout number too. Return to PlotMaker. You should see the open text block as you left it. Delete the DRAWINGNUMBER stuff. Repeat the process, this time choosing "Layout Number". Cmd+A, Cmd+C, Back to Archicad, Paste.
A detail marker will look like this in Archicad:
The gibberish bits resolve themselves in PlotMaker, giving the location of the drawing you set as autotext reference in the beginning.