Press it…
Step back, run behind a wall and hold your ears 
Hi ciquta, ive wondered myself, google ai suggests it can be pressed to lock the tailgate and doors to save you using the keyfob if you are walking away from the car. I would think you do this when youve closed the tailgate already as it would be too high to reach when in the open position. I will try it later.
I’ve tried it and I can confirm it is to lock all the doors and tailgate.
it can be pressed to lock the tailgate and doors to save you using the keyfob if you are walking away from the car
if only that would work… my car doesn’t close automatically when I walk away with the fob
I performed several tests but no luck, does nothing under any circumstances of car open/closed and presence/absence of the fob.
Maybe you have to press it when the trunk is open?? Makes no sense.
(one thing I just found out: if you have windows open, once you press on a random door handle to close the car all the windows will close as long as you keep the thumb on the handle)
I close the tailgate pressing the red switch, when it is fully shut press the button and it locks all the doors.
To unlock the car I just touch any door handle, to lock the car again tough any handle.
If you press unlock on the fob, press it again and keep pressed, all windows should open, to close them all do the same but this time with the lock on the fob. Hope these work for you.
I have manual tailgate and nothing happens with that button.
I’m aware of the trick with the fob for the windows, I had it in my old 2012 DS4 but somehow in the past few weeks it’s not working anymore on my DS7.
Although I love this car I have to say it’s a hit & miss experience.
I have read in other threads that if your 12v battery is low on charge it can cause things not to work. The ds7 is demanding on a battery. My last csr, a 2015 cmax still had its original battery,at 10 yrs, the ds7 last about 5 yrs!. I just hope these disappointments are worth it.
My new DS4, has had 5 replacement 12v batteries in 18 months all under warranty.
Touching on the auto opening of the windows, it doesn’t work on my car if I press and hold the open button on the fob, but I haven’t tried pressing once to open the locks and then press & hold to see if it opens the widows.
Will give it a go later.
You’ll get used to the DS being VERY hit & miss.
Don’t think my DS4 E Tense has that small button on the tailgate ?
The car has auto proximity opening & locking so perhaps that’s why.
According to the user manual, there are two options associated with this button:
Normal locking (x1 button press)
► Press the handle on one of the four doors (at the markings) or the locking control on the tailgate.
It is not possible to lock the vehicle if one of the electronic keys is left inside the vehicle.
Closing the windows and sunroof
Maintaining pressure on the door handle or the tailgate control allows the windows and, depending on version, the sunroof to be closed to the desired position.
This operation also closes the sunroof blind.
Deadlocking
Deadlocking renders the interior door controls inoperative. It also deactivates the central locking button.
The horn remains operational.
Never leave anyone inside the vehicle when it is deadlocked.
► Press the handle on one of the four doors (at the markings) or the locking control on the tailgate to lock the vehicle.
► Press it again within 5 seconds to deadlock the vehicle (confirmed by the temporary lighting of the direction indicators).
…As a safety measure, never leave the vehicle without taking the electronic key
for the Keyless Entry and Starting system with you, even for a short period.
Be aware of the risk of theft when the key is in one of the defined zones with the vehicle unlocked.
My note: Deadlocking isn’t standard on every car. It depends on the trim level.
Yeah, my tailgate doesnt have this small button.
I guess its because of the proximity self opening/locking that car does have.
I also tried the opening the windows with the fob by unlocking the car with the fob and then holding down the the same button on the fob, it didnt open the windows, but I can close the windows by holding down the lock button on the fob.
Thanks Andrew!
does it says something about the automatic lock when you leave with the fob??
it’s really annoying, was supposed to be key-less
Keyless entry (proximity auto lock/open), can play up if one or both of the 12v batteries is low/failing.
It will also disengage if you walk past the car too many times without actually getting in the car and driving.
Theres a lot of info on this from the link below.
Select your car and language, then search proximity access in the search bar.
https://service.dsautomobiles.com/DSddb/index.html
Keyless entry can be disabled on the screen, and then the door handles and buttons become important.
I only use keyless entry and then the rest doesn’t matter. Only in the event of a failure as mentioned by @Jactac