Hello. I thought I would post an update on this, for anyone who is interested. To give a bit more context - the original issue was in my first project using Archicad. I have since carried out a few more projects and become more familiar with the software. For each new project I would simply copy the PLN file, and delete the old project elements - allowing me to keep all my layouts and views as I want without having to set them up from scratch. Anyway, on my latest project, I noticed that the 'mystery line' had persisted, and was still present in my roof plan!
I troubleshot as follows:
1. As before, the line shows on the drawing placed to layout. I did 'explode into current view' command. This allowed me to select and inspect the line. I found that the line type was dashed, and was pen 70 - associated by default with the beam reference line. I suspect then that this line is coming from a beam.
2. I set up a schedule looking for beams on the roof layer. There shouldn't be any, but there was one! The schedule reported a beam with length 0.00 and with no 3D axonometric preview available.
3. I used the 'select in 3D button' from the schedule. No dice. I got an error message telling me it can't select it because it is not visible in the 3D window. Well I tried changing all the 3D cut and filter settings as per the error message, but got nowhere.
4. I used 'select on floor plan' button from the schedule. This one worked! Whilst the plan itself showed nothing (regardless of using zoom to window, turning on reference lines, anything to try and make it display) the element properties panel showed that a beam had been selected.
5. I deleted the beam, and the line disappeared. Happy days.
6. Still curious - I undid this and inspected the beam. Turns out it was an empty custom profile that I must have made back when I was learning the basics.
I'm not sure really what the lesson here is. Don't have empty complex profiles I guess is one. It would be nice if there were safeguards against this type of thing, perhaps we shouldn't be able to create them in the first place, but who knows, maybe there is a use for this that I don't know about. Anyway, it took almost two years to the day to solve, but I am very happy now that I have managed to get rid of this cursed line!
Archicad 23 Solo, Dell Precision 5540, Windows 10 Pro, Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz 2.59 GHz, 32GB RAM