"Keres" is not normally used without an object. This usage implies a hidden object, most likely the listener, and thus has the most natural meaning as "A boy is looking for you inside."
I wrote in English "A boy is looking inside", which in English could mean looking inside from outside, but I used it in the context of "searching"...
I translated "Egy fiú ..." by "One boy ..." and got corrected. I thought since egy = one (the numeral), it could be used in this context. Is there a reason it's not possible here?