First Gen S&W 9mm autos can be inertia drop fired and that is why S&W went to the 2nd gen design,
Walther P38's and P4's can be as well so guessing the PPK series has the same fault when safety is off, this was fixed in the P5,

Also while not certain cant Series 70 and older 1911's drop fire if not on half cock ?
Pre 1972 Ruger SA revolvers can also be drop fired if not on half cock,
NAA mini revolvers can be drop fired if the hammer isnt on the cylinder notch between rounds too.

We just dont know the details or firearm involved.