Another possibility is that it's being started by
anacron. That could be starting it very shortly after boot.
The first step is to figure out if it's starting at or near boot or when you log in. The easiest way to check that is probably to create a second user account, disable automatic login, reboot your system, and log in to the second account to see if the script is running.
If it is running on login, check not only your
.bashrc but also your
.profile and your
/etc/bash.bashrc and similar files. If those don't start it, it could be started by something related to your X session; the simple way to check this is to do a non-graphical login, either by starting your Pi in console mode or by ssh'ing in from your second account (after a reboot and having confirmed that it's not running before you log in to your
If it is being started on boot I might start by creating a systemd unit that checks whether or not the script is running and records that, and then move that unit around in the dependency graph to narrow down where it's starting.