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So Person A says:
どうしたんですか？Person B replies:
忘れ物をしたんです。
I'm not understanding した in this sentence. 忘れ物 means something left behind or forgotten. So I translate 忘れ物をした as did forgotten-thing. Which makes little sense. So how am I to translate or make sense of した's use here?
する here is a light verb, which means you basically have to memorize which verb should be used with which noun.
In Japanese, 忘れ物 is a noun which is always used with する, and 忘れものをする is a very common way to say "forget/leave something". You have to memorize this.
English also has verb usages which are almost unpredictable for learners. In English you can say "do the dishes" to mean washing dishes, but Japanese people never say 皿をする. See this question, too.
忘れ物 is a noun, so you have to add する to make it a verb.
You can take it together to mean: "I forgot something."
A similar example is 食べ物, literally translated as "thing to eat" or "eating-thing". So in the same way you would say: 食べ物を食べた。 - I ate something not I ate something to eat.
According to the dictionary, 忘れ物 can also be used as a verb:
http://dictionary.goo.ne.jp/jn/238200/meaning/m0u/%E5%BF%98%E3%82%8C%E7%89%A9/
So 忘れ物をする means "to forget something, to leave something behind".
I agree with you that literally the expression doesn't make sense, but it is listed in the dictionary.
