I'll answer everyone together:
@Rickster I'll check, i didn't look if the rail is clear. However i have opened it while standing by it and didn't hear any banging, scraping or anything. but i'll double check.
@Gordon_R Hmm ok so this answers my question about how the gate's positional markers are just metal welded on the gate about 10cm long towards either side. I imagine they are detected by the optical sensor? I will have to check. I am a bit DIYish but i have been known to set a few things on fire so i'll tread carefully when i check

Also see my reply at the end...
@ikpat I have one of those too - so see below cause my answer with
@Gordon_R might apply here ...
@Mzezman That battery was part of 3 others i bought for a friend. Both his 2 died after 1 year in an alarm, s i'm thinking possibly this one is toast too...
Ok so about the sensors.
I did do the following, i don't know if its an RTFM issue or not:
1. When it happened the first time, I had unlocked the clutch and the gate was open. I re-engaged the clutch and did a "Setup Wizard". It tried rolling the gate forward and back and ended with "Collision Detected."
2. It did the issue a few times.
3. I then unlocked the clutch and CLOSED the gate, re-engaged the clutch and re-ran the "Setup Wizard" and it opened and closed, then open and closed again and said "Complete".
It then worked fine for 4 days until it began its original issue about again...
Not sure if this info gives you anything more to go on?