In what circumstances will the police not warn the victim of a stabbing while the victim was at a laundromat under surveillance?

Similarly, in Davidson, officers had a laundromat under surveillance because someone had been stabbed there the night before, and there had been three other stabbings at the same or nearby laundromats. (Davidson v. City of Westminster, supra, 32 Cal.3d at p. 201.) While the laundromat was under surveillance, the victim was stabbed. Although the police were watching the suspect, they did not warn the victim. Still, the court found that the officers did not create the peril to the victim because "[t]heir conduct did not change the risk which would have existed in their absence[.]" (Id. at p. 208.)