Because some elements are similar in their spectral signature, there may be instances in which the quantitation software mistakenly reports an element that is not present. This can happen when lines from an element that is present in the sample overlap with the spectral region the instrument is using to quantify a different element (that is not present). Examples of false positives: Pt in W Alloy, Mn in Cr alloy, W in Ge alloy. Also, if the background signal is higher than the quantitation software expects, this may be interpreted as elements that are not really present. See, for instance, the following examples: Sn shielded by paper and Cd shielded by plastic. If an element is present which is not included in the suite of elements being analyzed for, false positives can also occur.
Fortunately, by looking at the spectrum, one can usually tell if there are elements that are not being analyzed for, or reported in error.