Enterprise Says I “flooded” Their Car Based On A Wet Paper Towel, Now Billing Me $15,195. Am I Actually On The Hook For This? Loss Type: Water/flood/submersion.

Location: Texas, US
Long post, but I'm genuinely lost here and really need advice.

The rental: Rented a 2025 Mazda3 from Enterprise, April 16–20, 2026. Returned it on time, no issues flagged at the counter, car ran perfectly fine the entire time and there was not even a single scratch.
Two days later, I get a call saying the car was "flood inspected" and they found water in the spare tire compartment. Their proof? Small blue test cards they stuck around the trunk, spare tire well and floor mats some partially wet, some not. It had been raining the day I returned the car, so obviously there's going to be some wetness around door sills and trunk edges. I never drove through a flooded road, no warning lights ever came on, nothing.

The bill: They sent an "invoice" for $15,195.40 (Actual Cash Value $22,827.40, minus a $7,925 salvage credit, plus fees). Their paperwork lists the "Date of Loss" as 04/19 — a day before I even returned the car. Their own photos have handwritten notes on the windshield dated 4/23 and 4/27, and their official damage valuation report wasn't even generated until 06/09/2026 almost 7 weeks after I returned the vehicle. So there's this huge unexplained gap between when I dropped it off and when they actually did whatever inspection led to this claim.

The other wrinkle: I booked using a company discount code (a university code, I'm not an employee there, I used my friend's code) that includes a damage waiver. When Enterprise first called about the flood thing, they told me they were fine covering it under that damage waiver. Now they've flipped and are asking me to file through my own insurance (Geico) or pay out of pocket, presumably because they realized I wasn't actually eligible for that coupon code or associated with the university.

Where I'm at:
I have Geico but I'm worried filing a claim tanks my premium for something I genuinely didn't do.
I have literally have not drove the car in the water and any flood. I don't think a partially wet paper towel and a week-plus-long gap in their timeline is solid proof of anything.
Has anyone successfully disputed one of these "flood damage" claims? Does the timeline gap actually matter, or am I reading too much into it?
Any advice, similar experiences, or things I should be doing right now would help a lot.