Rex was now in the worst situation possible because he thought that Alex had heard everything that he had said.
Furthermore he himself wasn't aware of the fact that he had said something and even if he did, he didn't know what exactly he said out loudly.
That was why he hesitated to say anything else when Alex asked that which took him by surprise all of a sudden.
All he could do was just think of an excuse that could actually convince Alex for the time being.
That was he would be able to escape any further interrogation. Thus he averted his gazes and started to think hard when Alex asked him once again.
"Tell me, what did you mean by Unnecessary worrying?"
Alex was also unaware of anything that he said. All he heard was those two words and nothing else probably because he was also tensed.
But with just those two words he could deduct something but wasn't completely sure about anything as such.