Found these play money coins at a 1905 homestead. They were about 5" down and two feet apart. The back is hard to see, but it has a big 5 on it and a ring around the edge. They appear to be aluminum, and are very thin and light
No such thing as a stupid question! Your detector may be set up to ignore them...or any other metal.? After a while you get used to the sounds...try testing it on different types of metals.
As much as I hate to I dig all foil targets unless I find alot of cut up cans. Have found a few vintage tokens that are Alum,And you see MDoob's play money. You never know.
Toy cash registers used to come with play metal coins, usually aluminum since it was cheap (this was in the mid 1970s). Shortly thereafter they went to plastic coins. You can buy play money by itself today, complete with plastic coins.
A few games also used to come with coins, also aluminum. Most just use paper money for play money now.