It could be possible that they left the device in the machine. In most cases it is quite obvious which cables are not the original ones and put in afterwards.
If you follow them you should be able to find the possible tracking device.

If you want to be sure to cut of all power, you should disconnect the negative lines, connect them to one of the terminals on your kill switch and then connect a new cable from the other terminal of the kill switch to your battery negative.

If you want to measure which line is draining your battery just put an Ampère meter (something like a Fluke 325) around them seperately. The one draining the battery should give quite a high current without contact key on.

edit: If you have any radio or other devices on the forklift with memory, you can chose to not cut those with the kill switch.