How is OS X able to authenticate to my router even when the disk is removed? I removed the hard drive from my laptop (MBP 9.1, mid 2012) and started recovery mode. Once I entered recovery mode, the laptop was connected to my router (secured with WPA2), even though I had not given it credentials. I pulled out the RAM (in case the creds were in volatile memory), put the RAM back in, and lo and behold, it was still able to authenticate to the router.  Is OS X storing keys from my keychain in NAND flash (where it stores the UEFI)? If so, I thought that the CPU couldn't write to the NAND flash (ROM -- read-only). The hard drive was disconnected the entire time. If this is happening, how do I securely erase the NAND?
The information is stored in your NVRAM and potentially in a keychain on the Recovery HD. You can clear it like this: How to Reset NVRAM on your Mac

See this for more details: How to prevent storing the WiFi password on the recovery partition?