Was on the Star last week (Trieste to Barcelona) and since it was an all-EU cruise (along with Montenegro, which would not dare risk its tourism recovery by imposing testing requirements), there was no testing during the cruise. Only at embarkation (or up to 3 days before if done independently) and then 10 days later, only for those who required it to travel home. There were rumors of cases among passengers (but there always are). If true, these would only have been caught if those people self-reported symptoms or had a fever detected by the face recognition getting on or off the ship in port.
There were definitely cases among the staff and crew, most notably the cruise director, who disappeared after Day 2, and we only ever heard from the assistant CD after that. We were told that the ship had just experienced a peak of crew cases a couple of weeks earlier, and that more cases had been brought on board with new crew in the middle of our sailing.
Unless forced to by local regulations (like the Israel sailings), NCL is not interested in investing in testing for all passengers every few days. And I suspect that most passengers prefer it that way. But it is a huge gamble to mix a thousand people together from all over the world (plus hundreds of guest-facing crew) and then put a blindfold on for 10+ days.
Masking was supposedly still required on board for this sailing, but it was only mentioned in the Daily on the first day, and absolutely not enforced. And I'm sorry to say that there were anti-mask jerks on board who actively picked fights with people they saw wearing masks.