Hey borndem it could have been a trophy or something kat realized wouldn't blend in with the fauna and rock scape in the dump site and put away.
Perhaps those are the panties that ended up in the dumpster along with Lizzi's sweatshirt?
Or she was making an explanation of why Lizzi's DNA could be found in/on the pants she was wearing that evening? I also noticed how she's repeated that when Seth raped Lizzi, her underwear stayed on and 'he just moved them to the side' and then 'snapped them back into place', I thought those were very weird details, but coupled with her explanation of putting them in her pocket makes think she has good reason for detailing the underwear.