Was the snare in a position where you'd be able to see the perpetrators from any window? If it was, maybe you could set up a temporary makeshift CCTV to gather evidence. A simple webcam, computer, and a piece of free software called i-Catcher Sentry will do the trick. The software has change detection, so only records while anything is happening, so no need to trawl through hours of footage. It may not be illegal to set a snare, but it is illegal to intentionally trap domestic animals (and as always, the law is ambiguous, in that how do you prove its intentional), and the land owner may be upset about people trespassing on his land.
