Reading through that, it seems like Mendeley uses SQLite for it's local database.

There aren't that many SQLite encryption libraries around (generalising), so it's probably using either SQLCipher or SEE.

https://www.zetetic.net/sqlcipher

https://www.sqlite.org/see/doc/trunk/www/index.wiki

There's a reasonable chance the page size and passphrase is stored or cached on the machine. If someone (not me) takes the time and effort to trace through the application, it should be feasible to figure out what's going on. :)