If I remember correctly, the mod I'm looking for allows you to select any object or actor and see information on that object or actors source, i.e. "which mod this this thing come from?"

I'm hoping it also works on missing mesh signs

Maybe http://www.tesnexus.com/downloads/file.php?id=21862?

or http://www.tesnexus.com/downloads/file.php?id=16704

Both RefScope and FormID Finder work for missing meshes, but the yellow missing-mesh warning can't be selected with the crosshair so you have to select it in the console. If you're using RefScope, select it in your spell list, press the tilde key to open the console, click on the yellow warning thing so that its ID appears at the top of the screen, then press tilde again to close the console.

FormID Finder comes with an alternate version of Marker_Error.nif (the missing-mesh sign) that looks like a red stop sign and can be selected with the crosshair. If you install that, it'll work with RefScope too.