You sign off a statement at closing

If the loan was acquired as for 2nd home and you actually use the property for rental - bank can take the loan back at any time, and you'll be subject to criminal charge.

All lenders require this statement nowadays - because they do know some people take a 2nd home loan (for the mere benefit of 0.5% interest difference) but use the properties for investment.

They want a way that they can take action if they choose to, any time.

Plus, in this internet age, to find out a property is rented or not will be a matter of minutes.