The behavioral symptom most likely to have prompted a CT scan for a tumor in the right frontal cortex is motor skill deficits on the left side of the body. This is because the right side of the brain controls motor function on the left side, so a tumor in the right frontal cortex can cause weakness or difficulty controlling movements on the left side. Additionally, tumors in the frontal cortex can cause personality changes, irritability, emotional flatness, and problems with decision-making or concentration, but motor deficits are a specific and direct sign that often leads to imaging to detect such tumors. Thus, the most likely symptom that prompted the scan would be left-sided motor skill deficits due to the right frontal cortex tumor.