They may have been lost or had a reason they needed to get to a real road ASAP. I was up in Snowshoe RTT and got off trail once and ended up on someone’s private road by accident. I got out of there as quickly and quietly as possible. I get lost around my house on my dirtbike all the time. This also happened to me on my mountain bike, and I just had to get to a road since I was miles off course and getting really thirsty. Hopefully this was a one time event for you.

i will second this. In many high atv traffic areas there can be unwritten rules between land owners that posted signs are for other uses... hunting, etc... and atv trails are open for all unless specifically posted. for example, in the area we own land, posted signs are essentially ignored outside of hunting season, and between a few dozen landowners there is a huge network of atv trails that goes through all the different properties. during hunting season, we know to stay to our own properties.

now, you put up a "no atv's past this point" sign... well, then things change. that means you dont ride on that property, or down that trail unless you have permission.

so id suggest putting up a sign specific saying no ATV/UTV's first. might you get a couple random morons? sure. but that is going to happen no matter what outdoor activity people are doing. hiking, biking, hunting, atvs...

Definitely check openstreetmap. A lot of other apps will use openstreetmap data, so if there's a trail on there it will show up elsewhere. If it's your property you can delete the trails or mark them as private.