Passed is the past tense of the verb to pass. To pass often means to move past, which causes some confusion. Past means the time before the present and can be an adjective, adverb or a noun.
To pass can mean to sail past, to fly past, to run past, to hop past and so forth. If you have used a verb indicating motion already, then it will be partnered with past and not passed.
To test whether passed is the correct usage, substitute it with went past. If the sentence still makes sense, then passed is the correct word to use.