有一个例句我不知道怎么回事I found the door unlocked.I went into the kitch

有一个例句我不知道怎么回事
I found the door unlocked.
I went into the kitchen.
改成复合句就是
When I found the door unlocked,I went into the kitchen.
到目前为止我还明白,但是还可以改成是
Finding the door unlocked, I went into the kitchen.
这是为什么?什么情况下可以这样改?状语从句转换为现在分词结构又是怎么回事?
请帮我解答一下,谢谢!