I don't understand why they would say 'Iru?' alone. I thought you needed a longer sentence for a question.
いる？ is a colloquial way of asking "Want some?" (or "You there?"). Of course colloquial speech is free to break the usual grammar rules, but even in standard Japanese grammar both object and subject are optional, so that answering いる "I want some" (or "I'm here") would be a complete sentence. The only part that might be disputable in standard grammar is the omission of the question particle (usually か, but also の), which has been replaced by the Western question mark (marking rising intonation).
So no, you don't need a longer sentence for a question, certainly not in colloquial speech.