The ornamental design for a propanewarningsign for windows, as shown and described.
FIG. 1 is a front elevational view of a propane warning sign for windows
showing my new design, with the rear being flat and unornamented and with
the broken line showing for illustrative purposes only and forming no part
of the claimed design; and,
FIG. 2 is a top plan view thereof, with the bottom being a mirror image.