"Spot" feels very specific to me and retlh feels very general to me, so I'm less inclined to really view them as the same. However, given your English sentence, there is a very good chance that I would translate it with this Klingon sentence, so I'll add it to the accepted translations.
Area and spot are not exactly the same: an area can be much broader than a spot. However, area next to and spot next to are pretty much synonymous. Spot next to might in the right circumstances be more likely to be used for smaller locations than area next to, but the given sentence is telling someone to watch a location next to a rock, which isn't going to be very big in any case.
Now, if the sentence were telling someone to watch over a huge field next to a large rock, spot might be the wrong word. But there's no sense of that here.