***Mary's worrying where Jesus was and what had happened to him cannot possibly be sinful.***
I agree with you in that, if it is sin, it is no great sin. It is a sin born of love. But look more closely at the passage...
"Every year His parents traveled to Jerusalem for the Passover Festival. When He was 12 years old, they went up according to the custom of the festival. After those days were over, as they were returning, the boy Jesus stayed behind in Jerusalem, but His parents did not know it. Assuming He was in the traveling party, they went a day's journey. Then they began looking for Him among their relatives and friends. When they did not find Him, they returned to Jerusalem to search for Him. After three days, they found Him in the temple complex sitting among the teachers, listening to them and asking them questions. And all those who heard Him were astounded at His understanding and His answers. When His parents saw Him, they were astonished, and His mother said to Him, "Son, why have You treated us like this? Your father and I have been anxiously searching for You." "Why were you searching for Me?" He asked them. "Didn't you know that I must be involved in my Father's interests?" But they did not understand what He said to them. Then He went down with them and came to Nazareth, and was obedient to them. His mother kept all these things in her heart. And Jesus increased in wisdom and stature, and in favor with God and with people."
Don't you hear in his words a gentle rebuke of their obtuseness in requiring him to explain this? "Once here, did you think I should so quickly leave? Let ordinary worshippers be content to keep the feast and be gone; but is this all you have learned of me?"
What He says they should have known, He must have given them ground to know. She tells Him of the sorrow with which his "father" and she had sought him. He speaks of no Father but One, saying, in effect, My Father has not been seeking me; I have been with Him all this time.
(I have taken portions of this from the JFB Commentary)
"Don't you hear in his words a gentle rebuke of their obtuseness in requiring him to explain this?"
I'll admit that that would be one possible interpretation, depending on tone, body language etc. etc. which unfortunately we don't have available.
I think, however, that a rebuke of his parents by a 12 year old would be tantamount to a sin on behalf of the child. Remember that according to the law and custom, a 12 year old was still very much tied to his mother's apron strings and he would normally only be about his father's business when he had attained the age of 13.