Join the roommates in their desperate hunt for the missing TV remote by selecting the right word.

I'm not entirely sure where the remote went. It ___ be under the couch cushions, or maybe the cat hid it again.

The correct answer is could.

We use could, might, or may when we are guessing about a present situation but aren't completely sure.

Since the speaker says "I'm not entirely sure" and offers another option ("or maybe the cat hid it"), "must" (certainty) and "can't" (impossibility) are too strong here.