"I'll leave them for some passerby to find."
That passerby is DEFINITELY getting merc'd after he finds them, like the good Samaritan driver in Better Call Saul.
This is volume 5, chapter 3. This chapter technically ends a little ways into chapter 3 of the LN, but the progression of events is slightly different to the manga, and it's not that far in so you might as well start at the beginning of the chapter.
It's one of those aesthetic choices the author made at the beginning of the series and then never brings up/mentions again. It's like how he said Kuro's true form is that black dog thingy, but after she transforms the one time it is literally never shown in the series.
Bit of a late response but it seems like you've gotten to the point in the novel where Eden's been introduced but she hasn't really been expanded upon. If you're curious about her motivations/backstory