“Where is Zayd?” seeks information and does not itself assert a fact about his location, so the question as such is not a proposition.
“Close the door” is a command. Its purpose is to direct an action, not to assert the existence or nonexistence of a state of affairs.
“The door is closed” does assert something that can be checked.
An intelligible and complete expression is therefore not necessarily a proposition: the decisive criterion is whether truth or falsity can be predicated of its content.