Parasitic drain comes from a micro processor in the light. It is found in lights with memory modes. If the power was shut off to that processor, it wouldn't be able to remember the last mode.
That is not correct, memory mode does not requires any power drain (Check EEPROM), the reason for power drain is usual UI (user interface) handling. But it is a microprocessor that is responsible for the drain, while it waits for the user to press a switch or turn a ring.