Where are you guys noticing the belt dust collecting when you have belt slip?

Every since I installed the 3.5" pulley and 111.5" Dayco Poly-Cog belt a couple weeks ago I have noticed a lot of belt dust and suspected belt slip as the boost gauge will fluctuate between 7.5-9 psi on a wot run. The dust is thick on the new idler pulley bracket and top of the water pump. The other locations which seem odd to me are on the underside of the stock idler pulley and the top right corner of the alternator (as you are looking at the alt from the front). Just wondering if it is slipping on other pulleys besides the blower.

 

I added the metco 100mm idler pullies and the magnuson upgraded tensioner and have had no belt slip at all since and kept the same belt I had.