You're not hiding the fact that you did a subject to.
Get that out of your mind right away. If in your mind you're "getting away with" something put that out of your mind too.
If you know for a fact that there is a payoff clause, why in the world would you be unethical and try to cheat the other party to the contract? Would you want people to do that to you?
The thing is with banks...they don't enforce that little clause in their contract. They could and they know they should but we're talking a lowly bank employee here and there are thousands of loans.
It takes several months for it to "dawn" on them what has happened. Months in which you have faithfully paid on time. At the first sign of trouble they will excercise the clause. But usually they will not.
While you don't want to notify them of the sale, don't try to hide it either or do unsafe things like not have insurance.