It could be anyone, just anyone, who might go through your system. A relative. A friend. A date…. What if you get hit by a truck?
Veracrypt is open source, easy to use, bullet proof (was just audited) and free:
https://veracrypt.codeplex.com
https://sourceforge.net/projects/veracrypt/
https://en.wikipedia.org/wiki/VeraCrypt
http://lifehacker.com/windows-encryption-showdown-veracrypt-vs-bitlocker-1777855025
