What’s the subject?

What’s the subject for the verb suppose?

Now, yer mum an’ dad were as good a witch an’ wizard as I ever knew. Head boy an’ girl at Hogwarts in their day! Suppose the myst’ry is why You-Know-Who never tried to get ‘em on his side before. . . probably knew they were too close ter Dumbledore ter want anythin’ ter do with the Dark Side.

Answer

The subject of "suppose" is "I". It's been elided from the sentence. This is quite common in informal speaking and even in writing. "Hope this helps" = "I hope this helps".