Are there any metals that metal detectors can't detect?

Metal detectors work by detecting conductive metals. So it will detect a cast iron pot, but if the iron is in the form of iron oxide (rust) it won't see it. Lead, nickel, gold, and zinc are relatively easy to find in metal form. However, uranium is usually found as an oxide because it is too hard to maintain as a pure metal because it reacts with the oxygen in the atmosphere to oxidize.