If you see "Unknown Wait" for a hold, the title has expired or been removed from either BPL's or a partner library's OverDrive/Libby collection, and you won't be able to borrow it. Libraries may keep expired titles visible in their collection and use holds to help decide whether to purchase more copies.
You'll also see "Clean up hold?" for these titles, which is just a friendly check-in about canceling your holds on them to free up your library card for other holds.
You can then place the same title on hold with any of BPL's OverDrive/Libby partner libraries that still own the title. If you're still on the title's wait list when the library decides to purchase more copies or add it to the collection again, your hold will start moving normally.