In Fahrenheit 451, after finding the green bullet, what threat did Beatty announce?

Beatty threatens to find the person on the other end of the radio.

Montag and Faber make arrangements to hear each other through a special kind of seashell radio.  Montag calls it a bullet because it is about the size of a .22 caliber bullet. However, the “green bullet” is actually a small radio receiver and transmitter.

Faber wants to be able to talk to and hear Montag from anywhere, so he can guide him...

Beatty threatens to find the person on the other end of the radio.

Montag and Faber make arrangements to hear each other through a special kind of seashell radio.  Montag calls it a bullet because it is about the size of a .22 caliber bullet. However, the “green bullet” is actually a small radio receiver and transmitter.

Faber wants to be able to talk to and hear Montag from anywhere, so he can guide him through his subversive activities.  The bullet is similar to an earwig in our world, a tiny two-way radio.

Montag placed the green bullet in his ear. The old man inserted a similar object in his own ear and moved his lips.

"Montag! "

The voice was in Montag's head.

"I hear you! (Part II)

Faber’s plan is to give Montag directions on how to interact with Beatty and others from the safety of the radio.  Unfortunately, the way Montag uses the radio gives it away.  Beatty realizes that Montag has the bullet and hits him, trying to knock it loose.  It falls out and he picks it up.

Beatty switched the green bullet off and thrust it in his pocket. "Well--so there's more here than I thought. I saw you tilt your head, listening. First I thought you had a Seashell. But when you turned clever later, I wondered. We'll trace this and drop it on your friend." (Part III)

Beatty says that he saw Montag tilting his head as he listened to Faber.  He became suspicious that Montag was talking to a revolutionary.  He threatens to find the person on the other end and capture him.  Montag is already in serious trouble.  His wife called the firemen on him, accusing him of having books.  At this point all Montag can do is kill Beatty and run.

Montag uses his flamethrower on Beatty and takes off to find Faber.  Although Montag never wanted to kill anyone, Beatty knew too much and was too suspicious.  Montag could not risk Beatty finding his way back to Faber and the other book people.