Around 04:40 pm CET Official server (Nov 30), got one tapped (no sound) despite helmet / ballistic vest.
Location: North West Airfield close to Barracks (south west of the airfield)
Same thing happened a couple days before (despite being well protected - one tap (no sound))
Server name: DayZ EU - DE 1354
Address: 195.82.158.69:10100
Server version: 1.15.154337
Required game version: 1.15.0
Map: Chernarusplus
Mission: DayZ
Players: 60/60
If it's not a chetaer, which is possible, then the same thing happened to me back in the day, although on a little bigger scale because not only did I get one-shot, but also my buddy, precisely at the same second. In your case it could be a very precise shot, because bullets travel faster than the sound, so you might as well hear nothing and fall dead on the ground.
Same server, another day (Dec 15, 7:40 pm) close to the toxic zone: I found all the gear to enter the toxic zone after days of searching. Logged in today close to the rify ship (hidden under a tree) and got an instant one shot seconds after logging in a hidden position (no steps could be heard nearby). This server is full of hackers. Its pathetic.