When asked at a press conference whether the apparent trip was planned, Guardiola - who believed Mendy to still be in Barcelona - responded: "Wow".
He said: "Wow, I didn't know". I didn't know it.
The French full-back posted on social media platform Instagram that he was at Hong Kong International Airport and when that was brought to Guardiola's attention, the manager signalled his surprise by swearing live on camera.
"His injury is not easy, meniscus external, and it's not easy", Guardiola said.
"He said he was going to Paris but Hong Kong is far away, I need to download Instagram".
"I have to install Instagram because I don't know".
Asked if it would be okay for Mendy to be in Hong Kong, Guardiola laughed: "No, definitely not!". "He needs to re-adapt, if you start training in two days you are fit, he has to handle the effort we are putting on him - he has to be careful". "Yesterday he was in Barcelona".
"I was just joking with my Uber driver, I don't want no problems Pep", he tweeted.
This is the last time City will play after Liverpool before their title rivals get to go second in a month of Sunday games in March. 'Liverpool are exceptional. I don't have the feeling (Jurgen) Klopp, (Virgil) Van Dijk, (Mo) Salah and Sadio (Mane) feel pressure.
That came as a shock to Guardiola, however, who had granted the France worldwide permission only to go to Europe.
"Thank you so much, but the Madrid supporters think they are", he added.
They also have a new striker, Gonzalo Higuain, who scored his first Premier League goals in a 5-0 win over Huddersfield last week.
"But then you count leagues, Barca, Juventus, Bayern, but I'm pretty sure you understand my answer better".