Depending on what kind of flashlight you have, you may encounter what is called parasitic drain. Electric or digital switches have to draw power from batteries to work, albeit in very small amount. It’s nothing to be concerned about really, but if you want, you can turn the batteries in the opposite direction when you are not using it.
To read further discussion of this subject refer to the thread below:
“Generally speaking, lights with mechanical switches don’t have what’s called parasitic drain. Lights with electrical/digital switches can exhibit PD. The switches have to draw power, albeit in small amounts, to be able to turn the light on, or off, as that’s how they work.
If the light doesn’t have a way to lock out the switch (like my Sunwayman D40A,) we can unscrew the battery tube a few revolutions to break contact. This is called locking out the light. You can also do this with your 6P, which employs a lock out tailcap in its design, but you can also do this with all of your other lights.