Parasitic voltage is not a rare disease but it can cause problems when present. The most common parasitic voltage problem is when a battery top surface becomes contaminated with certain materials and creates a connection between the positive and negative terminals, most common on top past batteries, the amount of voltage is small but can cause problems over time. Very easy to check with a volt meter, check between both post and the top as in very rare instances it will only be present on one terminal.The other place parasitic voltage can cause a problem is the cooling system. The main cause is the different metals in the engine create a voltage in the fluid. Here again easy to check positive test lead in the fluid the other to the negative battery terminal. There should be no voltage present.FYI A voltmeter should be RMS capable as most are today and be able to read milli volts. I use a craftsman that sold for around $20 on sale.