What are the dangers of walking around a world where no one knows your there
There are very many dangers for us; if scientists had any demand to talk in popular media they would educate about them
For the Plejaren far fewer having developed over time the tools and expertise to tackle them. However secrecy has dangers, being ignored one of those, deluding themselves another. A vigorous risk assessment of reality and of worlds and Earth, most of all their own world Erra, minimises risks for them. They have gadgets and technologies on a belt they wear. Another individual monitors them from afar and one of the Apparatus can make them invisible by changing Dimension apparently so they could even be walked through as well as gather more knowledge about us than even we ourselves know and be undetected, or perhaps it be considered a paranormal experience or maybe a religious experience.
Most of the dangers to the Plejaren are not from us therefore but from other humans not in the federation, who they monitor, or have monitored in the past, assessed is a more appropriate word, which to Billy they call the "3 or 4 groups", "those which you know about", and "those that our thoughts could not penetrate their ships to communicate with".
If the technologies were to fail though, then there are several dangers for them on Earth, or any world like Earth, hostile or not.
- One of them is our brain waves they say, and the way we think, which they are usually shielded from, which apparently makes them ill.
- Another is being stranded, it taking time to manually locate the individual, but those are not Plejaren Federation issues because they can map all life forms on any given world. Less capable humans from other worlds would theoretically have many problems becoming stranded here on Earth or on similar worlds.
- Were there ever any problems like Semjases accident, then someone; probably a team would act quickly to retrieve the individual. They are always monitoring each other and there are backup systems and procedures.
- Other situations include Easter island, Giza Pyramids and Tunguska Event, each with their own individual scenarios where the extratterestrials either broke the rules, abandoned the rules or never knew the rules, with varying outcomes. The Plejaren Federation have had their own series of mistakes too in earlier Earth epochs with for example the creation of Adam and Eva, and the formation of Christianity.
We're also not expecting them nor has it ever been a fuss through out history for us on our side; so it's the same principle as a species of bird on an isolated island which has never seen humans before, happy to make itself comfortable until it feels threatened which it won't do. There's a space treaty which prevents certain activities, and before you ask, The High Council has a handle on time; so they'd know if a random race were planning something out of a science fiction movie for Earth long before they were even aware they were going to, making it more like minority report than war of the worlds, in real life it is not like either, actually few of these movies are like reality because they all break the creative natural laws which are unbreakable in real life. Also there are no random races as you can imagine from this principle, the Plejaren already know which ones at which times will reach the space conquest stage at which point a probability of contact then becomes real.
It's like the analogy that your odds of winning the lottery increase exponentially if you actually play.
Another analogy is someone walking through a dark scary wood alone in the middle of the night; what does it matter if it's dark and scary, your alone so entirely safe. It's when the others realise its safe, that's when it suddenly become unsafe, because now we know their secret we can disturb their unassuming safe passage. The Plejaren probably have their technicians run risk assessments based on the various new stages of intellectual information as and when it's about to roll out, if that is what your question is. When we reach the hyper technology stage they'll run another risk assessment, and when we reach the fine-material sciences stage they'll run another risk assessment, when we begin the space conquest stage (when it becomes private sector and broadly used popular) they'll probably go and run another risk assessment. Actually they've already done all that, it was just written to illustrate the progression of time and evolution, they have already discovered all the things we will in the future, the only differences are the very niche specialist and rare achievements, and because our population is 16 times higher than theirs we really ought to discover a few new things along the way.
Long amount of text, but this is how we can then come around to the Roswell Crash, where they were obviously not prepared for a secret military installation on such a world to be experimenting with all kinds of different types of radiation and directing it at the sky to try and bring down one of their vessels because to them just a hundred years before on their last visit they had little more than horses and carts. But quickly as I said, back on their home worlds the various technological adjustments are made to their flying and other devices. It's a Consciousness thing, and the rapidity and greatly accelerated rate of our Ageing compared to theirs is a factor for them and us in this way. Extraterrestrials don't expect to find very rapidly ageing humans on any world they encounter, this can be a danger to them if they don't identify it soon enough, because Earth humans despite looking in appearance as mature as a Plejaren, would only ever behave like a child with them, using extraterrestrials for political gains, project fear campaigns and military funding etc.
Apparently the Plejaren have covered Earth in warning telepathic impulses,  so that any extraterrestrials coming here do not think they are dealing with a fully matured homo based being because they won't be, their experience with us won't be as pleasant as it was for them on their last visit to a genus homo based world. Earth is one of only a few cases like this in the universe and that is why Nokodemion is here.