If this has been brought up before now, I apologize. I haven't seen it posted..
Using Occam's razor principle, the body was probably wrapped in what was close at hand and such item may have been used for a common practice in Texas in the fall and winter, and was now available for another use in hot weather.
In Texas, many private pools and spas ( permanent type hot tubs, but probably not that soft side thing) are covered with tarps during the fall and winter.
Large or custom- shaped pools usually have custom fitted pool covers in my part of the woods, but many people just use a very large tarp weighted down on the ends. An in-ground spa or a heavy fiberglass spa located outdoors could also easily be covered with a tarp in the off- season.
The tarp would definitely have pool and spa chemical residue on it. This could be another reason why the pool water was obtained. IDK.
Also, another thought on the red tape. Anyone involved in health care has access to Biohazard tape, which is always red in color and is used to seal items containing bio- hazardous material. IDK if this will come into play in Alanna's case, but wanted to add another type of red tape for consideration.
IMO,
Maria