Thermador documents F5 as a “hot cookware near the control panel” detection — but it can trigger even when the cookware in question doesn’t feel hot to the touch, since it’s reading heat near the control strip specifically, not the whole pan surface. A pan sitting too close to the front control edge is enough to set it off.
Moving the cookware away from the panel edge and letting the zone reset usually clears it. If F5 keeps appearing with cookware positioned normally in the center of the zone, that’s no longer a positioning issue and is worth a diagnostic visit.