In The Tragedy of Julius Caesar, who did Cassius think knew about the plot to murder Caesar?

There was a lot of nervous tension among the conspirators
in Act 3, Scene 1. Popilius Lena, a senator, comes up to Cassius and
says:

I wish
your enterprise today may
thrive.

When Brutus questions
Cassius about what Popilius just said, Cassius
answers:

He
wish'd today our enterprise might thrive.
I fear our purpose is
discovered.

When there are so
many people involved in such a dangerous conspiracy, there is no way of knowing who else
besides the conspirators themselves might know about it. For example, Brutus finally
gave in to his wife Portia's pleading and told her precisely, though not onstage, what
was going to happen. Who else among the conspirators might have confided their secrets
to wives, friends, or others? As Benjamin Franklin once wrote: "Three may keep a secret
if two of them are dead." And who else among those wives, friends, and others might have
passed on all or parts of the secrets to yet others?

It
turns out that Popilius is truly a well-wisher and does not intend to alert Julius
Caesar to the assassination plot. Popilius approaches Caesar as both Brutus and Cassius
watch them closely. Then Brutus tells Cassius:

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Cassius, be constant.
Popilius Lena
speaks not of our purposes;
For, look, he smiles, and Caesar doth not
change.