Finally... The HS89 was nowhere to be found because... I put it in the gate mechanism.
Sorry Stygians, but repairing the mechanism with the help(?) of the robot is so very confusing. It says that you should place the components near the machine, which I tried after havin forgotten that the new valve (?) and the HS69 where sitting inside. The solution is to inspect again the inside of the machine WITH CANISTERS IN YOUR INVENTORY, then you get an option to pour a canister, try to poor a canister, see that it leaks, then ask the robot.
If you imbecibly (?) ask the robot directly, he will not budge, wherever you drop the components on the floor. If you put the canisters on the floor or in the Empty Canister, nothing happens. If you do as above with the item in your inventory, it works...
Sorry guys, but this is very convoluted.
If a developper reads this thread...
1- I have searched 3 or 4 times for the Lost HS - all stuff stack , all non empty shelves or boxes, went through the maps, even those I have not visited after getting the HS-89
2- I am pretty sure I have got it at some point
3- I am pretty sure that I have transferred to a stuff stack at some point (somewhere in faceless territory or at the Cytosine outpost, for easy reminding)
4- I am pretty sure that I have not transferred it by mistake (it is indeed in a special inventory).
Nevertheless I could not find it back... so If you have a dvlper tool that can quickly scan through the save, let me know where it is (I LOVE to know where I made 3 or 4 times the same mitake). If you have a tool and find it, please let me know where it is. If you have a tool and can't find it, then you have a bug.
S.