Looks like you can reach that spot from the hatch in front of the seat? You might stick your head under there and see if the issue is the bond between the deck and the stringer. Thats an easy fix if you can get to it from the hatch
Ok I took a look from inside the front hatch and also from the hatch in the floor just in front of the console. I guess it could be separation from the deck to the stringers but hard to see the stringers. I can see a line of goop running from front to back which I assume is the bonding adhesive. What type of stuff would you use to inject back in there to tighten thing up again?