Descending out of the Boston Bravo, controller tells me to squawk VFR - thanks for the help, and over to local freq, etc. Then I get an alert on my IPAD that there is a plane same altitude just behind me. I descended quickly - so did the unidentified plane. Then I remembered reading about the "shadows" here or somewhere. A little scary until I figured it out. Hadn't seen it myself before. Of course, the controller most likely would have told me about it - I say most likely, because I have had experiences with them NOT telling me about an aircraft in the area after telling me to squawk VFR.
