Thanks, Wozza!
I ran into the same problem when I imported some MP4s then changed their Kind to Podcast while leaving the Album field blank. It created an "Unknown Podcast" folder containing the MP4 files in my iTunes Media Library, but I couldn't find the assets in iTunes under any category. I deleted the MP4 files and the "Unknown Podcast" folder without deleting them through iTunes since I couldn't find them.
When I added the Smart Playlist with Media Kind set to "Podcast", I noticed the assets that were created in the past. The problem now is I couldn't delete them, so now the assets were going to exist forever without any file to link to. They remained with a "!".
To fix this, I dragged the same files in. I changed their status to Podcast with the Album field blank, then I re-linked them from Smart Playlist. From there, I changed their Album data to a temporary name as well as changed their Media Kind to Movie, so they would be removed from the Smart Playlist. From the Movie category, I deleted them, and they are gone for good! They don't appear in any Smart Playlist marked with Media Kind as Podcast or Movie.
Thanks for sharing the Smart Playlist trick because this helped me resolve my dead-end asset issue.