Tried looking on Patmos? John was taking care of her as his mother before he was imprisoned and he was a pretty smart fella...just as you are.
He might have thought of the possibilities of veneration of Mary after her death and taken precautions that a son might for his mother.
I THINK that what you mean is that St. John might have taken steps to conceal the body of Mary from relic-hunters.
Your theory offers no explanation for the total absence of any CLAIMS by anyone to have relics of Mary. Fraudulent claims of such things are quite common in history. How do explain the total absence of any such CLAIMS, true OR false, when it comes to Mary?
Precisely WHAT PREVENTED even potential fraudsters from even claiming to have relics of Mary? They had every worldly motive (money, fame, tourist attraction) to make such claims, but they never did.