Have you had it scanned with VCDS yet? My money is still on a faulty instrument cluster...

 

Plenty of examples of weird cluster behaviour in the Octavia section:

- https://www.briskoda.net/forums/topic/441411-engine-managent-light-on-but-very-dim/

- https://www.briskoda.net/forums/topic/383236-dim-dash-lights/

- https://www.briskoda.net/forums/topic/378754-indicator-light-stays-on-dash/

- https://www.briskoda.net/forums/topic/378287-green-clock-with-arrow-and-left-indicator-light-on-dash-when-lights-on/

- https://www.briskoda.net/forums/topic/303790-vrs-2-fl-dashboard-lights-doing-some-wierd-things-help-please/

- https://www.briskoda.net/forums/topic/288040-orange-oil-light-partially-illuminated-with-lights-on/

Not with VCDS but a fault code reader, came up no faults. I also found out today after much searching online, that this was a common fault with Audi. Some owners had replaced the cluster ( expensive ) but several found it was due to condensation behind the dash, and others that it was a corroded or faulty connection somewhere. A lot just decided to live with it, as it doesn't show until headlights are on, and then only very faintly.