While clearing Kabanino (Кабанино) of Infected on the UK 5-50 Server I encountered two invisible infected.
The first of which I captured in this footage. Apologies for the darkness of the clip, this was due to a ShadowPlay bug. As you can see, the infected gave no audio or visual indication of its position, but was able to deal damage to me. I attempted to damage the infected with a couple of shots in the direction I believed it to be coming from, but saw no indication that I'd hit it. This could be due to the infected having no hit detection or me missing (This is more likely).
I was able to escape the infected by closing myself in a house. The Infected did not come through the closed doors.
A short while after, I noticed an infected corpse that I hadn't killed myself. I waited for five minutes, assuming there was another player in the area, but when I got close to the body, I noticed that there were zombie sounds coming from the close vicinity of the corpse. I checked the area to verify this, and there were no other infected around. But there were still footsteps and infected groans coming from the area mentioned.. I did not check if the body was lootable. I did however check if it could take damage, by putting several 9mm rounds into it. This had no effect and the sounds were still coming through, as if the infected was still roaming that particular spot.
I believe this to be a serious issue because, paired with the current infinite stamina that they have, an invisible infected is difficult to evade if a player is not around suitable shelter.