Metal detectors are useful tools, but they are not miracle devices. Just because the metal detector gives a meter number the same as a penny does not mean that there is a penny anywhere near the coil. The coil is a copper wire coil with electricity running through it, and this produces an electromagnetic field which distorts upon encountering various anomalies. There are lots of anomolies in real world conditions likes round holes in cast iron objects, wet soil, dry soil, bent iron nails, burned bricks, background ground mineralization, etc. Metal detectors will see a flattened steel bottle cap as a coin, even though it is ferrous. It's complicated. One of the most important lessons I learned with my metal detector early on was that it lies much of the time. This was driven home one morning at dawn when I was a kid, and took my White's Eagle II out into a field at an old fort which had been plowed for the first time. About 20 feet into the field I got a strong and repeating signal. The decal on my metal detector indicated that a number on the VDI of 32-34 was a pulltab. Well, I was not looking for pulltabs! I went about 5 feet and got an identical signal. I'm not digging those pesky pulltabs! I kept going, and some salty and smart @ss old timer decided for some reason to follow along my footsteps into the field. He got the same signals I did except he dug them. They were both pristine stamped brass bugles from a Civil War era kepi hat. I learned a powerful lesson that day, and I could have had a pair of those beautiful insignias in my display of fort items. But I learned!
My point here is that there is almost certainly no penny in that cast iron object. Your detector is registering the shape of the object, the hole, and the shape of the hole, and giving you some data. Since you are mentally calibrated to coins, you are interpreting the data as coins, but it's not.
The metal detector lies!
