I have a switch that keep turnin on by itself.
Is there a way to track by log or something “what triggered the request” ?
Do you have any automation that could turn the light on? If so check if it has ran around that time. (hopefully you catch it before it’s ran as expected later)
No 
That’s the weird part!
Does the light turn on if ha is off?
What if you block smart life / your phone from accessing the network / Internet, does the light still Turn on?
The key right now is to try and isolate each part to see who the culprit is
My guess is these may be one of their newer devices with a non-flashable chip, hence no tasmota or other. Similar to what lolouk44 said, maybe you can do some traffic monitoring to figure out what the device is doing online when this happens. That said, at least you use HA and could get rid of those cloud based devices and replace them with fully local devices if you ever wanted to.
Of course, the solution brings a little embarrassment on my side… I’m using HADashboard (I have a tablet glued on a wall displaying all my devices in home assistant).
Sometimes, I need to touch it to “wake up the tablet”… And I most of the time, do this by touching it (while it’s sleeping) where - once tablet wakes - an automation button sits. My fingers seems to have caused the mystery .
This nevertheless is my most likely cause, but I also found that a script with a “Wake on Lan” turning on my computer might have also been the cause (which worries me because I never send a “wake on lan order” to my network.