Failing to verify existing password hashes with ruby BCrypt

I'm currently running into an issue trying to implement an authentication on Ruby on Rails against an existing database with existing password hashes. I do have a couple of existing plaintext passwords readily available, so I know a few plaintext/hash pairs to verify the process.

Passwords are stored as $2a$12$xxxxyyyy, so I would assume that they are hashed using the BCrypt algorithm (can I really safely assume that?). When I now try to run

password = ::BCrypt::Password.new("$2a$12$xxxxyyyy")
verify = password == "known_plaintext_password"

I always end up getting false for the verification. When I try to manually generate a hash using the same salt with the original password using

bcrypt = ::BCrypt::Password.new("$2a$12$xxxxyyyy")
::BCrypt::Engine.hash_secret("known_plaintext_password", bcrypt.salt)

the result actually differs from the original hash, though I would expect the hash function to return the same hash when called with the same input password and salt (again, can I safely make this assumption?).

I do not know which platform was used to implement the original hashing process, but I strongly suspect it to be implemented in .NET.

Am I doing something wrong here when trying to verify the password? I personaly would expect a pepper to be used on the original password before generating the hash, can I safely take this assumption, or might there be anything else going on? Are there possible incompatibilities between different implementations of the BCrypt algorith?

Since there has been no real answer here, and I figured out the solution in the meantime I will add my own answer for reference if anyone else encounters the same problem...

It was indeed the case that the passwords were hashed using a pepper (or secret salt, if you prefer that terminology). Fortunately I was able to get the secret used, and after adding it as a parameter in the devise model authentication against the legacy data was working.