[a1r]

The writer & comedian Richard Herring devised a fairly complete set of rules for this, calling it Consecutive Number Plate Spotting (CNPS). It's amusing, https://www.richardherring.com/cnps

[eesmith]

Found his blog entry when he found #999. From 19 years ago(!). https://www.richardherring.com/warmingup/26/7/2004/index.htm... . Took him 20 months, and that was with notes about where he saw cars with upcoming numbers.