1. Game cameras. They are purpose built for this kind of need. Your strategy should be to identify/establish clear routes in and out of your local area, and then set up an intel network in cooperation with neighbors. If you can network basic cameras, you can feed the input to a face/person recognition software. Say once every 30 seconds its sends the frame to an AWS instance or similar, and if it sees a person it rings the alarm/email alert.

Siesmographs and other such sensors can detect and track vehicles causing vibrations, some can be laid in roadway.

There are also some sensors that will detect only humans, via the ammonia our sweat contains, but I dont know if those ever made it to the public market, they were tippy top secret for the longest time.

2. Dogs. There is a landowner near me who is responsible for $5M+ of equipment and facilities. In addition to cameras and such, he has trained dogs that roam around at night. If you are already a livestock handler, adding "kennelmaster" to your job is a great way to solve your problem.

PS. There was a wave of this in my home state in the 80s. It was a combination of cults practicing, and uncommon diseases killing off livestock, which wild animals then tore apart to eat only what wasnt rotten.