Apologies for the vagueness of my memory.
I started using Bitcoin in mid-2015. I believe I installed Bitcoin Core at that time but got bored waiting for it to sync the blockchain, so I instead used MultiBit
Last week (April 2017) I decided to migrate off what is now called MultiBit Classic, since it's no longer supported. I upgraded Bitcoin Core to the newest version, waited for it to sync the blockchain, and was surprised to see two transactions from October 2016, both involving the address 1QKcPYdiDfPgNH4dYnC99uZqTszU4gndnn.
Dumping my wallet shows this address was generated back in 2015, and is marked as "reserve":
[private key deleted] 2015-04-29T18:42:36Z reserve=1 # addr=1QKcPYdiDfPgNH4dYnC99uZqTszU4gndnn
I'm sure these transactions have nothing to do with me. I wasn't using Bitcoin in October 2016, and I don't believe I ever performed any transactions at all with Bitcoin Core. What's going on?