Agnostic (in general) might toss a prayer up just in case someone is listening, or ask for some sign of a creator or the spiritual. They might have thoughts of God and be conducting a sort of search for God. Those thoughts and their search and their acceptance or possible desire that there is a God would have as much an effect on their behavior as atheist or theist.
Or it is possible that they just don't care. Perhaps they were raised in a cult that taught them that god was going to kill them any day now if they didn't believe the right thing. And perhaps they grew up having nightmares about that. Perhaps having grown up they have decided that there is no way to know 100% sure and they, therefore, don't feel a great need to find the answer having spent so much time worried about it growing up. Perhaps they have accepted that they don't know and that they don't need to know. Perhaps they figure that if there does happen to be a loving god that said god knows they are a good person who does the best s/he can, that said loving god would understand and accept where they are spiritually knowing what led them to where they are. And perhaps they figure if there is a god who is not a loving god who would understand those things then it is not a god they would worship anyway. And perhaps they figure if there is not a god at all they still believe in being a good person and live their life according to their own moral compass, which is generally a pretty good one. Perhaps, just perhaps, they truly live their life not worried about it one way or the other at all. I mean, honestly, what is the behavioral difference between a theist and an atheist? What line is drawn *behaviorally* that an agnostic must fall on one side or the other of? We believe that both theists and atheists can be moral? Can be good people? Can be utter asses? Can lie, cheat, or steal? Can sometimes convince themselves that something is right when it is wrong? Can do good in the world? Can be good to the people they love? Can be bad to the people they love? Can be smart, intelligent and logical about some things? Can be blind about or not understand other things? I mean, really, what behavior belongs to a theist that cannot belong to an atheist? What behavior belongs to an atheist that cannot belong to a theist? What *behavior* is there that one must adhere to that the other can not that means an agnostic must either choose that behavior or not?
Some people believe in intelligent alien life. Some people do not. Some people accept that they don't know the answer and it does not change their behavior in everyday life. How is a belief in god, a disbelief in god, or admiting that one does not know or worry about the existence of god any different? Just because it is a defining point in a theists or an atheists life does not mean it has to be a defining point in an agnostics life. One thing that is defining in my life is the type of relationships I enjoy. What is defining in my life is not necessarily defining in yours. By the same point just because belief or disbelief in god is defining in your life does not mean it is defining in someone else's.
I don't think anyone can define someone behaviorally as theist, atheist, or agnostic. Even if the behavior is going to church. I know atheists who attend church. And I know theists who do not. Even if the behavior is prayer. I know atheists who pray. And I know theists who do not. There are too many variables in life to be quite so defining, in my opinion. :)
Best,
Jackie