Rancher,
Welcome to CPF. It's a great group.
Here is my $0.02 from the peanut gallery, I suggest that you consider getting (or borrowing) a night vision goggles or monocular.
Have somebody with a regular light turned on going around on the ATV in the suspects area, and you have someone hoof it around with the NV equipment. Bet you'll find your suspect somewhat quicker.
Lessee now, 50 head herd, minus 1-2 head @ month, that is about a 4% attrition rate, in three months, you've lost 10% of your herd. Nice if you can afford it, but it sounds like a lot to me.
Please keep us posted on your progress.