The Raspberry Pi LXDE autostart configuration file can be in different places depending on the version of Raspbian you are running.
The LXSession autostart config file including it's location are discussed here and the configuration file is /etc/xdg/lxession/<profile>/autostart. So where's <profile> on your Raspberry Pi?
The default profile name is LXDE and the profile can be overridden by passing a parameter to lxsession. To see the profile setting on your system enter the command:
sudo ps -ef | grep lxsession
If you see the -s parameter then the default profile name is being overridden by the value that follows. In most cases this is LXDE-pi.
The LXSession documentation refers to logfiles but not where they're located. Here are possible locations: