Bonjour Lucy,
Chris and Maarten are both correct: there will be no -s if the subject is the singular formal "vous" and there will be an -s / -es if the subject is the plural "vous".
If the question was the multiple choice version, the only possible correct option is "vous vous êtes caché". If you were expected to write the answer, the different answers given above (with an -s / -es and without) should be accepted.
I hope this is helpful.
Bonne journée !