This has happened to a small handful of other users as well. With your iPod plugged in do they appear in your PC's Trash Bin? What happens if you reset your iPod Touch? Do they still take up space after that? To reset the iPod, press and hold both the Sleep/Wake and Home buttons together long enough for the Apple logo to appear.
If not, try simply unsyncing and resyncing all of your podcasts by removing the tick next to Sync Podcasts from under the iPod's Podcasts configuration pane in iTunes.
Otherwise, a full restore of the iPod may be necessary to free up this extra space.
Since you are looking under the iPod in the sidebar (instead of on the Podcasts tab over to the right), I'm guessing you manually add items to the iPod (by dragging items to the iPod in iTunes), instead of using automatic syncing. Is that correct?
On the iPod, do the podcasts appear in Podcasts? (How do you play them on the iPod?)
In iTunes, do you still have at least one of the podcasts remaining in your iTunes library? If so, do they appear in Podcasts, when you select it under LIBRARY? If you manually manage content on the iPod, I realize they may no longer be there in the iTunes library...
When look "under the device" in the sidebar, and select Podcasts, you say they are not there on the list to the right. Have you tried looking in the other categories under the iPod, such as Music, to see if they appear on those other lists anywhere?