Start out cheap and work your way back to the traps. Take a few crackers and for 1 night leave them in a few places. Count how many you put down and see if the same amount is there in the morning actually you may want to leave them out for a whole day. See if any are missing. That will confirm if something is there. It is fairly common for mice to look for dry warmer places in the winter. If the pup is going nuts, there definately was something there. Also, look behind appliances (washer dryer hotwater heaters or anything else with a flashlight to see if you see droppings, not only will it confirm the if but the how big.....
One little note. Maybe she is remembering when the squirrel was there. My Sparkle is a light addict and the lazer no longer works, hasnt for months but she saw it and went CRAZY....hey its an idea.....