Not all double pane glazing are same. It is possible your windows are double pane, but only dry air in between. This doesn't do the job as well, and you may be feeling the draft created indoor side of the windows, not air infiltration.
With canned smoke, check around the window trim, around recessed lights (if you have them), around outlet/switch covers, around base molding. Having insulation done and house wrapped do not mean that points of air infiltration were taken care of. Without gutting the interior, blown in, dense pack cellulose gives the best value for air/insulation.
Plexi glass is expensive, and will not stay clear. Just cleaning the plastic will scratch it and dull it over the years.
If you have hot air furnace, cold spots in areas or between cycles could feel like draft.
The movement of air around the glass is convection and while argon does help slow energy loss, it won't be that much of an improvement.