Mendy had been in Barcelona on Thursday for assessment on his knee, on which he underwent surgery in November, with City's preferred specialist Dr Ramon Cugat.
Guardiola, who was speaking at his press conference ahead of facing Chelsea at the Etihad Stadium on Sunday, was informed by a journalist that Mendy - who remains sidelined through injury - appeared to be in Hong Kong, judging by a post on his Instagram Story.
"Wow, I didn't know", Guardiola said, per the Manchester Evening News. "What? F***, he's a lucky guy".
When asked about Mendy's post, Guardiola said: "Wow, I didn't know". He said he was going to Paris but Hong Kong is far away, I need to download an Instagram account.
"I have to install Instagram because I don't know". "You have to know!" "No it's definitely not OK for him to be so far way".
Mendy cleared up the issue on Twitter, insisting it was a joke.
Guardiola's comical reaction to Mendy's whereabouts soon vent viral with the hashtag #WhereIsMendy trending on social media. This was in reference to the Instagram post, which was a video in which he included the driver.
Mendy is known as a prankster in the City dressing room and became a fans' favourite last season because of his antics on social media while recovering from his serious knee ligament injury.
The Frenchman later claimed he was only joking, but not until after he had caught his manager Pep Guardiola by surprise.
The former Barcelona boss warned the full-back in August that he needed to forget about the internet and focus on his football as he looked to put an injury-hit first season with the English champions behind him.
Of course, Mendy tweeted "oops" in response, but later showed he was trying to take his manager's advice more seriously.
'It's a final for us, and we have to play it in that way, ' he said.
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