AKA: Law And Order: Ghost Victims Unit
Despite Mulder's motivation being (supposedly) the loss of his sister, The X-Files doesn't really do kidnappings. Oh it's done it once or twice but for the most part, not counting alien abductions, actual kidnappings are pretty rare. There's probably a complex psychological reason behind it, with kidnapping being just a little too real and dark, or maybe they just don't want to have to keep checking in on their victim in the script, who knows. Hence we all know that when they do kidnapping, it's cause Mulder wants to make it personal.