I started a transfer over AirDrop of a 4GB xcode dmg file. Something happened in the middle of the transfer (3.5GB) and I now have 3.5GB of space missing from my computer. I have restarted the computer and deleted the file in the downloads folder. I tried using grand perspective to find large files but I can't seem to find the location of the temporary airdrop file. Where are these temp files located so that I can delete them.
I believe that airdrop transfers directly into your downloads folder. Should anything happen to interrupt the transfer, you will likely end up with a file called "transferred_file.txt.download", the file is renamed to remove the .download upon completion. If you have any .download files sitting there and not changing in size, they are failed downloads and can potentially be restarted (if they originated from Safari), or removed.
You can test this by transferring a new large file, and watching the file get created, increase in size, then get renamed.