Timo said:
The secretive Ramura species has multilayer defenses against being remembered. One level is the pheromones that prevent biological creatures from forming longterm memories of encounters with this species - so if our heroes don't make careful notes and cross-references, it might be that they will never again check out those recordings they made of Kellin, as they will not recognize their relevancy.
The other levels are technological, though. The ships of the species are cloaked. And as Kellin tells us, the Ramura people carry further technologies that prevent tricorder scanning beams from penetrating, or transporters from locking.
However, while Kellin suggests she wore a personal cloak that rendered her invisible the first time around, she doesn't wear one on the Doctor's examination table. And thus a human eye or an ordinary camera can record her visual image - and so can the visual sensors of the EMH. Why should those sensors be any worse at seeing things than, say, Chakotay's eyes or a video camera are?
Let's not get confused about "technology vs. biology" here. "Technology" isn't one big lump that reacts to things the same way in its entirety. "Technology" can be things as different as pulleys, warp cores and film cameras.
One can always manufacture technological sensors that are so similar to human senses that if the sensors refuse to work, then the human senses fail, too. A starship and her EMH might very well have such idiotproof visual sensors (say, those video cameras).
Cameras and the like could only be blocked if Kellin remained cloaked all the time, or if she or her automated protective technologies somehow managed to sabotage every single bit of video memory, film or other recording media aboard the Voyager that has her image on them. Kellin says she performed such sabotage the last time, by planting a computer virus - and apparently this virus erased the memory of the EMH as well. But unless Kellin uses her personal cloak, there is nothing to prevent the Doctor from seeing her in the first place.
Timo Saloniemi