Given that both client and guide apparently realized Cecil was collared you would have to have a disregard for common sense which beggars belief to argue that Palmer wasn’t aware of what happened to it.
“I was devastated. I could not have seen the collar at night. We would never shoot a collared animal,” Bronkhorst told them. “I was devastated, and so was the client, we were both upset, and I panicked and took it off and put it in a tree. I should have taken it to Parks, I admit that.”
Why was the lion collared, was it to trace his movements?