In going through some old backups I found an old bitcoin wallet.dat file and decided to download Bitcoin Core to load it and see if anything was in it, or to just see if there were old transactions listed that I could daydream about having now

Opened the file, and oddly there was a pending received transaction from December 2013 that is sitting unconfirmed. This transaction showed up immediately before I downloaded any of the blockchain yet (waiting on that to finish, estimated time ~week...)
If I search the received address or the transaction ID I find nothing.
Also, I have no recollection of where this transaction could have come from and have not used that receiving address for anything else.

Couple questions:

-If this isn't showing up anywhere (blockchain.info, for example) where did it come from? Could it be an intentional double-spend just to mess with me?
-can this transaction be rebroadcasted if the network has no record of it?
-do I need to have the entire blockchain downloaded to get my client to rebroadcast this transaction?