I have seen more than one buried plastic tank float out of the ground with high groundwater.
Also, they may have a tendency to collapse if left empty.
My 2 cents.
Yikes! I did not think about the tank collapsing..... would you leave an underground tank empty?
I hear ya on the float, I have can't be sure till I test it.... would you agree with my test procedure....also, typically, what type of floats are used in these underground storage tanks?