I found the following info on the web. I guess it's not supposed to be visible when new but mine just showed up:



"Pilot Power/Pilot Road Technical Information

Anti Static strip

When a motorcycle is in use there is a build up of static within the machine. It is necessary to ensure that this is dispersed during usage.
In order to facilitate this there is a carbon black strip built into the tyre during manufacture. This strip is moulded into the tyre before curing using carbon, as this is a good conductor of static electricity. This strip is practically invisible on a new tyre, but can appear as the tyre wears, especially if the usage of the motorcycle is not high. There is no detrimental effect on the performance of the tyre either before, during or after this strip has become apparent. The positioning of this groove is slightly ‘off centre’ and can be seen in the diagram below."