Roman coins have shown up over the years in many places in USA, school yards, soccer fields, homestead areas, and fields. So many were brought over in the past century that unless they were found in situ with an archaeological dig for proof of rough time the coins was lost, it most likely was a more recent drop in the past 50 years or so.
Just in recent times, since the 80s a lot of our serviceman detected in England and brought back many roman coins, also collectors buying bags of them is big in this country, so it most likely was a purposely put in the field event, lost by someone who had one with them for good luck or some other reason, but as a coin that would have been used in old times, nada..
Constantine coins appear to be quite common, but have no idea on the different ones out there. There are several ancient coin experts on the forum.
Don