Unless the pet's id was in our own system (been in as a patient) we still had to phone one of the databases who then attempted to contact the owner (as opposed to some magic direct access).
Whilst it's still the 'best' system not all chips wordlwide are ISO compliant and some older pets may have been imported with such other chips. Also though rare i have experienced a couple of chips over the years that failed and a very few that migrated from the ideal insertion site.
Lastly I'm sure readers have improved but even with our professional models we had to swap out for quality fresh batteries regularly to get good performance… tested with at least two readers before assuming it wasn't chipped.
pgk