Why is mentioning loops a bad sign? The question is simple, and asks for only checking one value; that's a simple answer that, for one value, takes linear time, which is the best you can do (without assuming the array is sorted, but expecting someone to ask you if the array is sorted turns it into a trick question, rather than a real world one). I mean, yes, I'd prefer them to go to a library call for that, but I wouldn't expect them to infer additional complexity from how I stated the problem.
Sure, if someone were to start asking questions, finding out that we're doing this operation a lot against the same arrays, so it makes sense to rebuild them into a set, or we're doing it against only sorted arrays so we can do a binary search, or whatever, is great...but that's more interview technique, not actual coding skill. You're clearly asking a contrived problem, have provided no additional context (no business requirement is "search two arrays to see if a number exists in both"), so it's not really fair to assume a prospect will treat it as an ambiguous requirement that needs further refinement.
This is really the heart of the issue; you're not testing for basic coding ability, which is what you're thinking you're testing for; your own criteria is testing for whether they assume you're trying to trick them. Talking it out isn't testing for coding ability either; I've met people who could say "You would search through each array to see if you can find the item in question", and then literally could not code that correctly, in their alleged preferred language.
If you want to measure whether someone knows to clarify business requirements, give them a business style requirement. "We have an interface that allows users to provide a list of their favorite foods. We want to provide a way for users to compare their own list of foods with that of a potential friend, and see what they share". Okay. That leaves the underlying data structures up the developer (as they are in the real world), and describes a problem they can ask questions around.