I have read a few reports of waterproof torches (namely P60s, both the original $$-brand and Solarforce alike) leaking water at the tailcap switch rubber boot, especially when submerged for prolongued times and the switch operated, repeatedly even, under water to change modes.
Seems to be a common problem when usage extends the ubiquitous kitchen sink/ bathtub/ shower tests, which they pass just fine.
Ah well when I dump mine in the bath I leave them in for a couple of hours and click them on / off a few times. I’ve only tested the TN31 and the V11R recently like that. Both have magnetic mode changes / brightness controls.
The V11R misted up inside a tad but not too badly. a quick clean of the seals and re-seating of the o-ring and all is good again.
They are both NOT diving lights though and I’d never loan one out for diving use.
My TK35 clone had no O-ring at the tail. There is a place for it but it was missing. Later I realized that the O-ring from the tail was used at the head of flashlight (It has oval shape if you don’t stretch it) so I placed it where it belongs and placed a new round one on the head part. If it is clone it may be the same case… just unscrew the plastic tail and take a look there. If the O-Ring is missing, take a look at the one used on head. Strip it down and maybe you will see an oval O-ring like me