it's a common household item, i really doubt that stores have that shit on some kind of watchlist. if you want to be super paranoid about it you make a roundtrip where you visit a dozen different stores in the area and buy just a single container at each location and pay with cash.
They’d pull bank records if he was a serious suspect, and probably watch any retained security footage from his trips to stores.
This isn’t about anyone being paranoid, it’s about police trying to solve a disappearance.
body dissolved in his bathtub means it's simply a bathtub worth of water/sludge going down the drain, plus a little bit extra for cleanup afterwards. pretty much the same amount of water you would flush when using the bathtub the regular way (to take a fucking bath)
Your bathtub drain is not like your toilet drain. That’s why a little hairball from your wife will fuck it up
Edit: I want to add an example case.
Adam Montgomery was a homeless heroin addict driving around in New Hampshire when his five year old daughter had a bathroom accident in her car seat, because dad was nodding off and wouldn’t stop anywhere for her to use the bathroom. So this guy killed her by repeatedly punching her in the face.
The car broke down literally minutes later, so he stuffed his daughter in a gym bag and took off, abandoning the car.
The police have been able to piece together every place he took her body. He put her in a cooler (frozen body at first, it was winter) in the hallway of his mom’s apartment building, he took her to a homeless shelter and stuffed the bag in the drop panel ceiling, he took her to his short term job at a pizza place and put the bag in a walk-in freezer, then eventually he rented a Uhaul to dispose of her body in Massachusetts.
They haven’t found her body, but everything else, they have records for. They cut the ceiling out of his room at the shelter place to run DNA, they have him on camera at all of these places, they have the texts from his phone when he made arrangements to get a friend to rent a Uhaul for him.
If people have the resources to pursue the case and you’re not in Chicago, they’ll find something.