According to the Tunnel's website, it appears that the perps did a middle of the night test run a few days before. I imagine they cover this in the actual show, but I've only watched the black car segment, don't speak Greek, and couldn't access subtitles.
"Residents of the area talk to “Tunnel” about strange movements a few days before the heinous crime in the maisonette in Glyka Nera.
“I was listening to a motorbike the previous days, which was standing for a few seconds at the corner of Parodos with Panagouli street. They drove around the block, returned and left. It gave me the impression that it was a big one. I did not see anything, I was just listening, because it was late, around two in the middle of the night”, said a resident of the area.
...
Another witness, who lives near the house of the tragic couple, spoke on the show about what had happened a few days earlier.
“We were also targeted. The bell rang twice at four o’clock in the morning. They were plowing the area fifteen to twenty days earlier. Someone would come and look on a bike, going slowly and wearing a jacket hood so that his features would not be visible.”"
All very odd, who is going answer their door at 4 AM? Why would these people be attracting attention themselves & allowing nosey neighbours etc to possibly get licence plates, vehicle & person/clothing descriptions of them by behaving so obviously?