Megumi is right.
One same event/action can be described from different view points with different type of verbs(transitive or intransitive).
Basically, when you use a transitive verb(like mitsukeru), you are more conscious about the fact there is a subject who did it.
When you use an intransitive verb(like mitsukaru), you are more conscious about the the event/action. "Who" did it doesn't really matter.
Let me just give you some example.
Suppose you lost your key, and your gf found it in her room. Someone is asking you about this event.
Was your key found?(Did you find your key?)
-Yes, it was.
だれが かぎを みつけましたか？
Who found it? -My gf did.
かぎは どこで みつかりましたか？
-かのじょの へやで みつかりました。
Where was it found?
-In her room.