to check f/t feeders i always squeeze at the hips and the head to check for cold spots, as those are the last parts of the body to thaw out (they're the thickest). maybe next time try thawing in room-temperature water after the fridge stage.
The chest of a rat needs to be included here bc the rib cage protrudes. So, ultimately this can be deceptive of the true internal temperature of the organs and tissues behind the ribs.