double checked on Linux (Ubuntu Studio 16.04 64bit) as well and [symbol ./( works, but opens the directory from where Pd has been launched. For example if the patch is in "/MyFolder/PdPatch", and you launch the patch with Pd closed, then [openpanel] will open there when using [symbol ./(
In case Pd is already open, the path when using [symbol ./( is "/home/UserName"