EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*EDIT*
Well, drat. Turns out, that's the icon for a reflected
Pipe Bomb. Many thanks to Codewarrior, for his research into what the individual icons look like. The Reflected Flare icon is similar, but there's a squggily trail of smoke, instead of the straight line shown in the screenshot. I personally blame the whole mixup on my sketchy memory of what the Reflect Flare kill icon looks like. The day of the Pyro update, Majin, myself, and a few others went into our private server and tested all the physical projectiles for reflection (except the Sentry rockets, which we later learned could be reflected too). Pipe Bombs, Rockets, and Flares all change affiliation to the Pyro who reflected them. Stickies remain the property of the Demoman who fired them, but if he blows himself up with a sticky that was just blown back at him, the Pyro recives credit for the kill.
The following picture is courtesey of Codewarrior, who delved into the game files to pull out the kill icons.
Pipe bomb, Sticky, Rocket, and Flare, in that order.