Update:
The disc is entirely blank, as all my burning programs report it as write-ready. The only logical explanation is that I sent it to someone by mistake; however, the only transaction I've been involved in as of late is selling a copy of Zelda Wind Waker for Gamecube back in March, but the Gamecube game cases don't allow for a DVD to be put in them.
Spoke with a police officer and got some advice. While he understood I was spooked out by the ocurrence, he told me not to worry about it unless more stuff starts to appear at my door. There's an incident number they can use if that's the case, and things are looking calm as of now.
Thanks to all for your concern and advice.