I don't see him having an underwear fetish as much as it is a "trophy" to him...meaning he keeps it as his reminder of the act itself and his power. I don't think random underwear would hold the same thrill for him.
With his DNA on the Pink Panther thong, it would indicate it either happened during or directly after a rape or he was reliving the rape later on. The latter wouldn't surprise me.
I hope LE already has been checking across the country for victims with any similiarities to these cases. I think there must be more victims.
I am very concerned about the owner of the particular thong in this case.
The trophy theory sounds very realistic. However, a nationwide search is too unrealistic IMO.