What is the worst the banks can do if they found out?
Assuming the bank are entitled to charge a higher interest rate if your home becomes an investment property;
Could they charge the higher interest rate retrospectively if they discover at some future time that this happened in 2010.
With investment rates typically 1% higher than standard rates, this could mean an additional €1K being added to your mortgage for each €100K owed, for each year they did not know about the change.
Whether realistically this would ever happen or not, who knows, possibly as you say they are happy to be getting payments at the PPR rate today, but they might not always think like this.
Personally I would be aware of this as a possibility and continue to keep my mouth shut.