Understanding MS SQL Server encryption and backups
OPEN SYMMETRIC KEY MySymetricKey DECRYPTION`  
 BY CERTIFICATE MyCertificate`

  
  notice - there is no password here.


Yes, there is. You opened the certificate MyCertificate somehow, didn't you? There could be several cases, but any of them ends up with a password:


you opened the certificate using a password (obviously, there is a password in this case)
the certificate is encrypted with the database master key and you opened the database master key using the DBMK password -> that's the password
the certificate is encrypted with the database master key and the DBMK is encrypted with the service master key. The service master key was in turn opened using the service account DPAPI storage key, which is encrypted with the account password -> and that is the password


In the last key there seem to be no password, but there is one: the service account password. The backup, when restored on a different host, it will lack the service master key encryption of the database master key and this will make the data inaccessible.

See Encryption Hiearchy