Edited

[cappykat]

Removed

[dark rider]

It sounds like the center weight of the shell is not properly supported and the sides of the surround are carrying the weight.  What surface is the tub mounted on, and are you certain it is level with no settling?

[cappykat]

Removed

[Jacuzzi Jim]

 Take the panels off and look for wet spots, you can also use a wooden dowel and work your way around the spa slowly poking the dowel in the foam till it comes out wet.  Work your way from top to btm as you go around it.  Probably will be a jet of suction fitting? 

[Bonibelle]

I think you should let the dealer do that. You are still under warranty and have looked for issues that would not be covered (loose fittings, drain plug).  The dealer says the panels are a design flaw, so it seems to be totally unrelated to your leak. If you have logged the leak problem with the dealer, you are covered. He can't wait to address it and say " Oh sorry your warranty ran out while I was waiting for a tech to get there."....I am sure Marquis will take care of this, the tub is an '09!