When Ponyboy wakes up in the ambulance, what does he think the siren noises mean? Why do you think he makes this assumption?

When Ponyboy wakes up, he hears sirens in the background. His immediate thought is that the police have finally found him and are taking him to jail.

Then I realized it was a siren. The fuzz, I thought dully. The cops have come for us.
[...]
I opened my eyes. It was dark. I'm moving, I thought. Are they taking me to jail?

Fortunately, Jerry is in the ambulance, and he explains to Ponyboy that the ambulance is taking Dally, Johnny, and Ponyboy to the hospital for various reasons. Ponyboy hears this, but he still does not accept the fact that the police are not there to arrest him.

"Are you taking us to the police station?" I was still a little mixed up as to what was coming off.
"The police station?" It was his turn to be surprised. "What would we want to take you to the police station for? We're taking all three of you to the hospital."

Jerry explains a bit about their injuries, and then he makes the comment that the three boys are heroes or have been sent from heaven. Ponyboy definitely doesn't believe that, and Ponyboy then explains why he would have assumed the sirens were police that would be taking him to jail.

Sent from heaven? Had he gotten a good look at Dallas? "No, we're greasers," I said. I was too worried and scared to appreciate the fact that he was trying to be funny.
"You're what?"
"Greasers. You know, like hoods, JD's. Johnny is wanted for murder, and Dallas has a record with the fuzz a mile long."

Jerry can't believe any of that, and he again reassures Ponyboy that the ambulance really is taking them to the hospital.

In chapter 6, Pony and Johnny enter a burning church to save the children trapped inside. Unfortunately, Johnny suffers a broken back after a flaming beam falls on him, and Dally manages to knock Ponyboy out before he can return to the burning church. When Ponyboy wakes up in the back of an ambulance, he hears the sirens and immediately thinks that the authorities have come to arrest him and Johnny. Ponyboy makes this assumption because he and Johnny skipped town to hide in the abandoned church on top of Jay Mountain. Pony realizes that they are wanted by the police for the murder of Bob Sheldon and immediately assumes that the police have found them. Fortunately, Jerry is riding in the ambulance with Ponyboy and explains that they are heading to the hospital.