Snape and the Marauders
We know Snape knows Sirius is an Animagus as of Parting of the Ways in GoF. We also know that he is aware of Lupin's lycanthropy and has been for a long time.
Simple, right? Here's the problem: When precisely did he find out about Sirius's (or James's, or Peter's, if he even knew about them prior to PoA/Peter's defection, respectively) Animagus status? The Ministry didn't have the information, the dementors at Azkaban didn't know, but looking closely at Parting of the Ways, Snape doesn't seem all that surprised.
I realize this question probably has no true 'answer', but I need to know what would be generally accepted because the setting of my current fic depends on it.