If ion spectrometry results showed positive for presence of a substance is it possible that it is wrong?

Ion Spectrometry can be highly inaccurate. False alarms can be made due to the detectors inability to differentiate between some other substance and the one looked for. This may have to do with whether or not the false positive has been programed into the detector - in order to weed it out.

NOTE: It's not really that ion spectrometry is inaccurate. It is extremely accurate. Where there is A LOT of room for mistake is in the interpretation of the data. The detector doesn't identify anything. It simply sees a signal at a certain charge to mass ratio. The mistake is human error (unless the machine isn't working properly of course). Ion spectrometry can easily distinguish two molecules that differ by only one atom.

Ion spectrometry works essentially by weighing the mass of the components of a mixture. You then use the mass to match with what you think might be in the sample. Here is where the mistakes happen.

Here is an analogy: You've been robbed, and somehow you know the exact weight of the perpetrator. The police arrest somebody and he happens to weigh the same amount as the person that robbed you. Does that make the person guilty? No! There are lots of people that weigh the same amount as each other. Does it mean the scale that weighed the person was wrong or inaccurate? No!

Determining somethings identity by its mass is risky unless you have other knowledge about what you think it is. Just like in my analogy. If you knew the robber's weight and also that he had gray hair and blue eyes and was wearing a purple hat, then if the guy the police caught matched ALL of those details, you'd be pretty sure you got the right guy. But by weight alone, it can lead to mistakes.