If the spa leaked below the jets (returns), which is where the check valve is located, the leak has to be somewhere else. Usually, unless there is a broken pipe somewhere, that only leaves the suction side of the plumbing.Your response really gave me pause. I didn't have any food dye on hand, but before going to bed last night, I turned off all pool power (put the pool in service mode). This morning, the spa is 3/4 EMPTY! So what is that telling me? Either the spa one-way check valve isn't working or the leak is on the intake side? At the start of this adventure, I replaced the diverter on the intake valve (which had been leaking at the stem). So I need to find some Mrs. Stewarts laundry bluing and try this again.
Is 3/4 empty the same level, or nearly the same, as the pool? If so, strong indication of the water flowing back to the pool through the suction valve. I see that at least 3 times a year, maybe more. Usually the valve is set wrong, but sometimes the diverter is worn out.