I cleaned again and again the glass and it seems to be working ok now. But i cannot say that it was any grease or filth visible that might of been causing the issue!
It could definitely be a hardware issue. I wouldn't blame it on hardware until you exhaust all software options though. Restoring the phone using iTunes/Finder and setting up as new without using your backup would be a good way to find out if it's software. If it completely stops the problem you can restore using your backup and see if it comes back. The 6S+ has a pressure sensitive display and I suppose it's possible to have an issue with the way it reacts to inputs...