I got to thinking about these portals and then something crossed my mind. These portals are invisible so that unless an elf knows about its whereabouts, they will actively have to search for it. Sometimes a story just calls for a little ignorance or forgetfulness to make things a bit more challenging for a character. But anyone or anything can use a portal, so how do I prevent say an African elephant from stumbling into Siberia? (hypothetically speaking of course as this isn't Earth we're talking about).

So I've decided that a portal can be seen or sensed or even smelled in a variety of ways, just not by senses as limited as ours, or our elves in this case*.
Like ultraviolet light. We can't see it, but bugs can (among others) so it makes sense to me to that the kidnappers, which are based on bugs, can see the portals that way. And panels 2 and 4, I've decided, is what the portals look like in UV.