The possessive form of victim is victim’s.
It is standard to say movements instead of movement in this context.
The target is a loaner? As in, either the target is someone who loans things, or the target is on loan? (“Need a victim for your next serial killing, but don’t want to make a long-term committment? Come on down to Evil Rupert’s Serial Killer Emporium and just borrow one! We’ll loan ’em to ya for the best price around, guaranteed, or the next one’s free!”) No, I am quite certain the writer meant loner, meaning someone who tends to be alone.
Add a comma after disappears.
Delete the comma after notices.
A serial killer has been monitoring his next victim’s movements for months. The target is a loner, the perfect target. One day the target disappears, and nobody notices but his would-be killer.
Still too many uses of target in such a short paragraph, in my opinion, but maybe it at least shows the serial killer’s obsession with his intended target.
(I must share this observation: WordPress’ built-in spelling/grammar checker says I should change serial to cereal in the previous paragraph, but only there. Think about this, folks…)