This is a known problem with Pentax's wireless flash system. Or rather, it's a common problem with all such systems. (See similar complaint with Nikon.) Even though the manual states otherwise, at least portion of the control flash fires during the actual exposure, and you can see it in the image, particularly if there are reflective surfaces or if your subject is close. It was less of a problem on earlier Pentax cameras like the K10D, but then people complained about range and reliability. Newer cameras solve those problems, but the trade-off is, apparently, a more visible control flash.

There is a solution, though. I bought a cheap piece of IR-transparent plastic and stick that over the built-in flash. Unfortunately the flash won't close with it on, which is a bit annoying, but it's very light so a bit of transparent double-sticky tape is sufficient. More details on this here: Would this IR-transparent plastic be useful for blocking unwanted control flash?. If DIY seems a bit much, there are products sold for this purpose, but I actually like my hack better, since it goes in a tiny SD-card pocket of my camera bag — and it doesn't look ridiculous in use.