The file can be found here:
%STEAM_HOME%\SteamApps\common\dota 2 beta\dota_ugc\game\bin\win32\tools\model_editor.dll
Looks like a call to a physics test program is made here with the parameter "-game" to be "HL3" and "-open" to be something undefined, probably the name of the object to be tested.
The file was recently updated (February 3rd, 2015)
I'm usually searching through those files after every DOTA2 update since the Source2 implementation and I didn't notice that reference yet but there are chances that this string is in there since the beginning and is just some legacy stuff.