If you buy such property and you plan on doing renovations or any type of work that requires pulling a construction permit the current open permit process needs to be closed. First, I would hire an inspector to find out if the tank was in fact removed or not, like you said, the removal may not have happened because it could cause contamination. Or maybe the owner realized it would be too expensive to remove after he pulled a permit.
Whether you renovate the property in any way or not, if you purchase this property it will be your responsibility to finalize any work related to that open permit.
I suggest you contact the construction permit office for the county this property is in, they can give you all the information you need and you can also try to contact the previous (or current) owner?? or whoever pulled the permit and figure out from there what you need to do.
Hopefully you didn't purchase yet, get all your information and then make an educated decision about buying or walking away. Just my thought.