I shoot alot of old, shiny, black sewing machines, and reflections are my nemisis. I've had both my face and camera show up in knobs and levers, really weird looking!
Anyhow, if, in wireless flash mode, the built-in flash is set to Off (Discharges the built-in flash as a test flash), and if it's showing up in your photo, then chances are that you need to recompose things. Even without any flash, your camera would likely show up in that situation.
Below is a shot I took using the AF360 as a slave (wirelessly -the flash was about three feet off to the side) in full PTTL mode, controlled by the buit-in flash which set to discharge as a test flash only. You'll see that there is a white box, a mirror a dark table, light gray brick and a black fireplace in the scene. I set the K-7 to M mode, f/5.0, 1/80s, iso 200. Ugly and dirty, but quick.
I find that with any scene that has really dark and reallly light subject matter (like black and white) it pretty tough to get both exposed properly using any auto exposure or PTTL systems. I'm sure that some equipment handles it better than others, but to a certain extent, it's the nature of the beast. Thsi is where going full manual and using a grey card can be helpful. Using the +/- compensatin settings on the flash helps too. Most of the time, when I use my flash wirelessly, I use it in manual mode (1/8, 1/16, etc.. power). I don't know if the Sigma flash will allow that.
"What's with the weird sign??", you ask. I've been playing with using it to illustrate the shininess of the machines.