Texas has you half way there.
Do this
Turn the pump off. Kill the power .
Fill the spa up, and turn that three way jandy in front of the pump to the off pos. The suction side of the not running pump you turned off, put it in the off pos with that 3way jandy. If the handle does not turn all the way then lift the handle slightly and turn the off flap toward the pump. Now the handle/stick is pointing at you. And off is facing the pump.
Next turn that pool spa return off the same way with that grey jandy valve. Face the handle toward you and make sure the off flap is lined up nice with the pipe from the filter.
Now the only place for water to go down is in the spa body, or the pool body. For the most part.
Let that sit over night, Make sure your pump wont come on over night.
Tex is saying its leaking from one of those valves I think it is too. Go ahead and order two jandy key jandy valve assemblies and replace the lids. Or plumb in a check valve on the spa return and suction. Let see how your test goes.