The lack of agent in a sentence could have several reasons. The agent may not be important at all, or is predictable on general grounds. The agent may be unknown to the speaker at this stage of speaking.
Topic-comment structure:
蘋果吃了。
Pingguo chi le
The apple was eaten.
NP+NP+V structure:
蘋果小明吃了。
Pingguo Xiaoming chi le
The apple was eaten by Xiaoming.
In Chinese, an agent can be added to the sentence as secondary topic without extra marking.
