In one town the Hacienda and local authorities went to the internet and found the pictures and location of vacation rental properties listed for rent.  They then drove around town and from the pictures the pulled off the web located the properties, and then checked to see if any taxes were being paid.  When they found no taxes were being paid, they estimated the rental income the property would have made over the past few years and sent the owners a bill which was quite high.  Needless to say the time, accountants, attorneys  and expenses the “Gringo” owners had to go through to resolve the tax assessment was extreme. It is best just to pay the taxes to avoid these problems.