My Panda had the exact same done by Thames, I didn't realise though until I changed the plates which by then it was out of warranty, it really annoyed me as you say the 4 holes are already there, no one seems to take pride in their work nowadays. Check your front grille as well because they also did it to the front with my Panda which annoyed me even more as that has 4 indents where to drill holes for the number plates, back in the early 80's when my dad started his career as a mechanic he worked for a Vauxhall main dealer & they had templates from Vauxhall for doing the holes for number plates on all the new models coming in, after that he worked in a body shop owned by 2 brothers which he told me they were really strict & took no crap as they said to him when he started if any car you do comes back you're out as they had a good reputation & after the jobs were done they'd inspect every car & if there was something slightly wrong they'd say sort it out it's not going back to a customer unless it's 100%, luckily that never happened to my dad