BJH laid out several possibilities that I agree with. Personally, unless there's evidence to the contrary, I'd take the first solution, followed by the second. However, this is harry Potter we're talking about and I suspect it might well be the third possibility. I would hope that the pensive memories would eliminate some possbilities, though if it is the third one and the perp is still polyjuiced, that could start getting problematic.
Author's response
Thank you. Polyjuice, glamour, memory spells, just different ways to say BOHICA!
Expect the unexpected.
GG