Having just been in that area last week I can only guess as to the cause.
To inspect that area is fairly simple and just requires about 10 minutes to open it up. The wheel well panel is attached with two brackets. Once the square drive screws are removed the panel can be hinged up at 90'. Place supports under each wing. Then there is a plastic panel that is attached with some more screws and sealant. Breaking the seal is the most difficult part of the process. I just used a putty knife to get it to release. Once the panel is removed you can see the area in question.
One problem is that below the 1/2 osb flooring is about 1.5 inches of insulation and then an additional 1/8 osb panel so even looking at this area from below might not be very helpful. This photo is from the cutout under the shower pan.
Best wishes for a simple repair.