Does an index continue to be used after it is dropped (Database Polling investigation)
As Peter mentioned, the query may still be using a far better execution plan - and since you know how to check that, I'd suggest doing so - but it is not using the index.

When you drop an index, it is gone. There isn't any way it can linger around once your DROP command returns successfully.

I feel like I should write more, but its really just that simple. Check your execution plan and you should find your answer pretty easily.