Parasitic battery drain

I have just come across this term & noted some reviews that measure this characteristic with other comments such as "I unscrew the cap to properly switch off".

I own two LED flashlights using single Li-ion 1865 batteries & each sits "ready for use" but in fact are used only occasionally. They seem to sit without noticeable battery drain for months at a time.

I also own one a Sky Ray 3XT6 which uses two 1865 batteries.
I have observed that it generally cuts out unexpectedly after relatively little use & I then discover typically one battery is "low" whilst the other is of the order of about 3.6V.

My question is why do the batteries not discharge approximately equally & is parasitic drain responsible for the low life between charges?

I too would be interested in an answer to your question.
I just bought the new Klarus ST11 and it has one of those side electronic switches to change modes and I am currently monitoring
if it will discharge my 18650's faster or not.
P.S. It is a nice bright light for an EDC

Uneven cell state of charge is usually a battery or charger problem. Most lights with electronic switches have some dark current to keep the circuit " listening" for input. Most lights with mechanical clicky switches (in series with battery) have no such drain. 18650s have gradual mechanisms that drain and reduce total capacity.

If the light uses a mechanical 'clicky' switch then parasitic drain won't be an issue as the switch itself causes a physical break in the circuit. Electronic switches however do generally leave some level of parasitic drain as the circuit is not disconnected but simply remembering that it is off. This can vary massively in amount, but in most major branded lights is very low to the point of virtually nothing. It is worth looking up exactly what it is on each specific light though if you are concerned about it.

The behaviour of the Skyray 3xT6 sounds like one battery is slightly degraded/more poorly performing than the other. That will mean that that cells voltage under load will drop low enough to either trigger the cells protection circuit or for the combines voltage of both cells to trip the lights own low-voltage protection (if it has it). I would strongly recommend not using those cells together and replacing them with two closely matched (in age and capacity, basically two the same) cells.