(written from a Production point of view)
Referred to in the first draft script of the Star Trek: Deep Space Nine seventh season episode "When It Rains...", a data-sifter was a type of computer program that could be used to remotely conduct an automated search through another computer's routing log. In the teleplay, Miles O'Brien described a data-sifter by likening it to a computer virus.
In the script's 2375 setting, O'Brien set up a data-sifter in Deep Space 9's infirmary and let it loose on the routing log of Starfleet Medical's main computer, sorting through the log in search of any trace of whoever had altered files within that computer. The data-sifter was used because Julian Bashir, whom O'Brien was helping, was interested in finding out the identity of who was trying to cover up the results of a series of scans that Odo had been subjected to.
Regarding the data-sifter, O'Brien told Bashir, "When it finds something, it'll contact us." This ability impressed Bashir and enabled O'Brien to leave the infirmary. However, later in the script, O'Brien asked Bashir (upon returning to the infirmary) if there was "any word from our data-sifter." Distracted by his work, Bashir uncertainly replied, "Something was beeping over there a little while ago." Moments later, the data-sifter was disrupted by a burn-back that the pair came to suspect was run by Section 31 operative Sloan.