How does Hamlet know someone is listening

There is nothing in the play to suggest that he knows he is being watched when he asks ''where is ur father?'' There r many ways to play the scene, & I've seen all of them at some point:

1) Hamlet does not know anyone is there, & is just asking a question of Ophelia. (This makes sense, because he does threaten the king is life a few lines later.Not a good idea if he knows he is being watched.)

2) Hamlet figures it out on his own. (He has been called the smarted fictional character ever. It stands to reason that he could figure out that Ophelia would not just be wandering around the courtyard alone with ''remembrances'' to return.)

3) Ophelia tips him off.

4) Polonius gives himself away. (This is the most popular. Polonius is a fool, & could make a noise or something. Shakespeare probably would have left a stage direction to this point thought if this was the answer.)

Anyway it is played, it must be remembered that Shakespeare did not specify which is the ''correct' answer, so really any of the above is acceptable.

Hamlet is suspecting a trick: That he is being watched by Polonius (Ophelia is father).

In his famous soliloquy, he is thinking out loud to himself.

ESP

or maybe hidden secutiry cameras