In Macbeth, who does Lennox believe killed Banquo?

In Act III, Scene 6, Lennox is talking with an unnamed Lord. Obviously some time has passed since Duncan's and Banquo's murders. Lennox was depicted as very young and naive when he was present at the time Macduff discovered Duncan's mutilated body. But by this point in time Lennox has seen a lot and heard a lot, and he has become more circumspect and sophisticated. He has learned the wisdom of speaking by innuendo, if he speaks at all. His guarded language not only shows his maturation, but by implication it shows that Scotland has become a sort of police state, not unlike Germany in the time of Adolf Hitler or the Soviet Union in the time of Joseph Stalin. Macbeth has become a terrible tyrant and has decided, since everybody hates and despises him, to rule by fear.

Lennox's speech in Act III, Scene 6 is full of verbal irony. He is saying the opposite of what he means, and his tone of voice would undoubtedly be heavily freighted with sarcasm. With regard to Banquo, he says:

And the right valiant Banquo walk'd too late,
Whom, you may say, if't please you, Fleance kill'd,
For Fleance fled. Men must not walk too late.
Who cannot want the thought, how monstrous
It was for Malcolm and for Donalbain
To kill their gracious father? 

Lennox is comparing the flight of Fleance at the time of his father's murder to the flight of Malcolm and Donalbain after their father King Duncan was found dead. He implies that Malcolm and Donalbain had to flee for their lives to escape from Macbeth and that, likewise, Fleance had to flee for his life to escape from the murderers Macbeth had sent to ambush his father and himself. 

Lennox obviously does not dare to say what he really thinks. He doesn't even dare to be too candid with the Lord he is talking to, but only to hint at his meaning. Lennox breaks off, fearing he has already said too much.

But, peace! For from broad words, and ’cause he [Macduff] fail'd
His presence at the tyrant's feast, 

No doubt everybody in Macbeth's kingdom is learning to talk in riddles or to say nothing. Macbeth has spies everywhere. In Act III, Scene 4 he has the following exchange with his wife.

MACBETH
How say'st thou, that Macduff denies his person
At our great bidding?

LADY MACBETH
Did you send to him, sir?

MACBETH
I hear it by the way, but I will send.
There's not a one of them but in his house
I keep a servant fee'd. 

The short scene in which Lennox has a conversation with the unnamed Lord is typical of the ones Shakespeare frequently uses to convey exposition to his audience. And this scene conveys a great deal of significant information. The Lord tells Lennox that Malcolm is living at the English court and that Macduff has fled there to join him. Both Malcolm and Macduff have reported such atrocities to King Edward that their report

Hath so exasperate the King that he
Prepares for some attempt of war.

This news comes almost at the very end of Act III. Act IV and V will show Macbeth descent and final ruin.