My answer didn't use gunpowder, posited air or rail guns, and it was posted 33 minutes before your question. Oh well.
My alternatives here answer the question of impact that disappears, but it probably fails your modern day setting requirement. Plus I'm thinking totally out-of-the-box and ignore dissolvable or untraceable bullet.
All these suggestions probably require longer exposure on the target that a mere gunshot would at current technology levels.
Long Range Acoustic Device (LRAD)
While termed non-lethal, LRAD makes people run into tear gas to avoid the pain. So the target may run off a high place and fall to their death?
If you managed to trap the target and then bombard them with LRAD for 5 minutes there would be hearing damage and possible heart, lung and internal injuries.
High Intensity Ultrasound
https://en.wikipedia.org/wiki/Sonic_weapon#Research lists lung and intestinal tissue damage (in mice), heart rate atrial flutter and bradycardia.
Just design an emitter without the safety protocols: Low Frequency Sonar at 184+ dB or a focused ultrasound beam of 1 mW/cm² SPTA.
If you can get at the target's bedroom while they're sleeping you can probably kill them with sound.
Misused Directed Energy Weapon
These would leave evidence but given nobody dies of this stuff at the moment, a coroner would be puzzled by the delivery if not the cause of death.
Fry your target with a real microwave or an ADS.
Frozen Blood Weapon
I love @SRM's answer, especially if you harvested the target's own blood first.
Liquid Nitrogen's boiling point is (−195.79 °C (77 K; −320 °F)) so store the weapon in a portable flask.
Easiest method would be for a street vendor to stab the target repeatedly with an icicle made of his own blood. The local Nitrogen Ice Cream cart takes a darker turn.
Alternatively we're back to custom air rifles (more likely air cannon) or rail/coil guns with mesh/foil sabot.
Surveillance and Counter Surveillance
All of these methods require a good understanding of the target's movements and habits. But if you're trying to kill someone in an unusual way, I suspect that's part of the process.