I don't really see your problem. To begin with, the suggested translation isn't in the continous (is finding), it's in the present tense (finds), and secondly, why would you think that the present ought to mean a constant activity? The basic use of the present is to refer to something that is happening now.
"The boy looks behind the couch and finds his sweater". If you think of it as a live commentary of the boy finding his sweater, it should make sense.
Out of context the sentence might seem a bit strange to a non-native English speaker, but "the boy finds his sweater" certainly does make sense :)
Many English speakers (in America) also often use the present tense to tell stories of things that have already happened (or worse yet, alternate between present and past tense :D). I'm not sure where it started, or whether it's a specific regional habit, or tends to occur in less-educated areas, etc.... But, where I grew up, it was fairly common to hear someone speaking of a past events in the present tense.
E.g., "We went to Grandma's house, then we realize John's sweater is missing. So we look for it, and finally the boy finds his sweater."
It's called narrative present or historical present in English, but it exists in several European languages and has been around for hundreds of years. It's used to give a sense of dynamism or immediacy to a story. My theory is that it became ubiquitous when the "reality" shows took over, as they use it intentionally and continuously for this reason. Before the first Real World aired on MTV back in 1992, I hardly ever heard people relate stories in the present tense. Wikipedia has an article about it in eight languages: https://en.wikipedia.org/wiki/Historical_present
In English, the context may be different but the syntax is sound. "She's out finding herself," or "He's finding himself in a lot of trouble these days" are both quite valid and common. So the sentence structure is still correct in English, even if "The boy finds/is finding his sweater" sounds awkward.
Just a heads-up for the Powers That Be here on the Swedish team: the practice section here in Verbs is being very accomodating of my misspellings. It accepted my "hitter" and later itself presented the phrase "hittar", "seljer" when it on a later screen showed "saljer", and much in contrast to other screens it doesn't seem to care one way or another about my painfully scattershot use of a/ä, o/ö, a/å. I tried to Report this as "Other Problem," but I expect that'll be a bit vague for the reviewer.
I'm trying to get the spelling right. Really, I am...!