So where is the dividing line, then? When does incidental observation (but stored for all time in a database searchable by the public, say) become stalking?

In particular, is it possible that no individual act of recording personal data observed in public would constitute an unreasonable invasion of privacy (whatever we choose to call that in law) yet the combined total still has the effect of stalking? Because that is the real problem that, IMHO, the law hasn't caught up with yet. In a connected, database-driven world, you can't judge everything in isolation, because it is often the cumulative effect that is the dangerous thing.