外れる and 外す: detached parts and missed expectations
Compare a part coming loose with removing it, and examine incorrect predictions and briefly leaving one's place.
A connected part comes loose
かばんの金具が外れました。 says that a metal fitting came off a bag. 外れる, read はずれる, presents the result from the detached part's side.
古い部品を外してから、新しいものを取り付けました。 says that someone removed the old part before fitting the new one. 外す, read はずす, marks the removed object with を.
Worn items and leaving one's place
眼鏡を外すと、小さい字が読めません。 says that the speaker cannot read small print without their glasses. 外す also applies to removing worn or attached items.
担当者は今、席を外しています。 says that the person in charge is away from their desk. It is an expression for being temporarily absent, not detaching a chair.
Miss an expectation or target
晴れると思いましたが、予想が外れました。 says that the expectation of clear weather proved wrong. 外れる is not limited to physical parts.
ボールが的から外れました。 says that the ball missed the target. から marks what it went off; 的を外しました can instead describe someone missing the target.
Does 席を外しています mean that someone is removing a chair?
Show the answer and why
In a workplace description of someone's availability, it means that the person is temporarily away. Interpret the expression as a whole.