Hastings is distracted by Poirot’s sexy… ear? What is he looking at?
Actually he’s being surprised and confused because this is the one with the mob tracking down the American chanteuse who stole some navy submarine plans and Poirot has rented a flat in this building in order to stake it out and protect the young couple, the Robinsons, who sublet the chanteuse’s flat, from the mob hitman who otherwise was going to kill them. Hastings doesn’t know that, though, and he’s utterly confused about what the Italian embassy, Japp’s stakeout, and the foreign dude lurking outside the block of flats have to do with anything, plus he’s really confused about why his boyfriend even wants to rent a second flat that he’s not even living in for a few weeks at all. He’s just spotted some PCs undercover as gardeners in the park and Poirot is taking him to the surveillance van.