It's not a phone until proven to be one. There is room in the battery for some power and an explosive.
It most certainly is a small electrical device, solidly built and heavy for its size. IT APPARENTLY HAS NO OWNER ON BOARD THAT AIRCRAFT.
It is a potential hazard, and it's not up to the crew to assess the odds.
Millions of small actions by the TSA every day find nothing. Just occasionally something is found. The odds are millions to one against, but try telling the TSA to go home.
I was presented with a Dangerous Goods chitty. A small amount of a substance I had no real training in handling was listed. It was decided by the dispatcher I could not take it. I was sorry, cos it was the chemical in the battery of a life-jacket from another aircraft.
There are a horrifyingly large number of people, including children, who have been left with no feet, no hands, and/or no eyes. Devices I read about last week are not much bigger than a mobile phone. One weighs less than my phone.
The odds are against, but the threat is real.