Forum user Bomoo noticed there was a gap when two side fairings were mounted onto the Service Module:

http://images.akamai.steamusercontent.com/ugc/261587080048769047/ADD009943D8FB6019D466CCCD4C674E981C3D810/

I suspect this came about from the collider rework, and at the time I was trying to avoid explosions from intersecting fairings. But perhaps I can just move the visual mesh to line up properly.