There are some plastic panels that fit inside the bottom of the bumper cover. These are intended to smooth air flow under the car. You need to check these panels are all intact -- they can crack due to road debris impact just from age (over the years I had a few fail on my Boxster and replaced them) and all screws are present and the panels are secured to the car.

If the bumper cover has been removed by the previous owner chances are the bumper cover was not installed correctly. I had my Boxster bumper cover removed once or twice by Porsche techs and we talked about the bumper cover and they both told me they can always tell when a non Porsche tech has been at the bumper cover. Screws in the wrong holes, screws missing, those panels are not installed correctly.

A particularly noted area is the screw at the corner at where the side marker lights are located. I no longer have my Boxster and I can't run out and double check my memory but I seem to recall a couple of screws along the top of the bumper cover in the area covered by the leading edge of the front trunk lid.