the policeman is another of johan's obsessive clients. the top clients at kamiya are all vampires too since they can donate more blood, and the only way a regular human could reach the top spot without dying in the proccess would be by using someone elses blood
just like tanaka abused her authority as a doctor to get blood for him, he also did that, though unlike tanaka the cop shows no restraint when it comes to killing people. he said he was patrolling the area near the clinic when johan collapsed on the way to get an iv, but he was probably stalking johan.
idk if johan is in on it, but i would bet that, in the least, the policeman is staking out the club waiting for people who might also be one of the top clients to cull the competition. johan might know but hes not really in a position to reject any blood he can get either way since vampirism seems to be a pretty serious condition
edit: tbh now that i thought about it, when tanaka says "that's how it is... i'm the more reliable blood source though" it is pretty apparent that johan knows and shows up to drain the bodies