Looking online, the accusative for masculine ich is meinen, not mein. Only the neutral is mein. I would have thought this would therefore be meinen Rock. What am I missing? Is it because it's using the "to be" ("is my") verb in this particular example?
Not just because it's "to be" here - it's true for all verbs. "Wo" is just the question word, the real subject here is not "wo", but "mein Rock". You can compare with English: it's "Where am I", not "Where is me".