Re non feretic metals;
your spot on Numpty, different grades of stainless.
Most people think stainless is stainless and wonderfull but as it is more expensive than mild steel it comes in different grades depending on the use and ability to keep the price competative. Then cheaper ferritic grades don't have much nickel or molybdenum in and can get rusty ie pet food bowls, nick nack **** out of India and china etc.
then non feritic 304 medium grade, non ferritic 316 best grade ie marine etc, but even with these grades storing water is a problem unless it is osmosis treated water.
The reason for my posting is being a stainless fabricator I thought id make my pick out of 316 non magnetic 6mm steel and the detector wouldn't pick it up as it was classed as non ferritic and non magnetic, ALAS it didn't work, it may not have enough iron in it to make it non magnetic but it is still a ferrous base and my detector squealed like a pig. lol
Good try hey !