Here, all sorts of things could happen to bring crimes to light. A worker might rat out his employer. A news crew could be filming some other story. A civil suit might involve disclosure of practices. The FDA might investigate the farm as part of some food chain issue.
After all, we've created a seriously complex system of laws and enforcement to handle stuff like this. Are we now saying that they aren't good enough? If not, why not fix them, instead of having people sneak around other people's property? And let's not forget, if you let the special group called "people who care about animal treatment" violate property rights, then you'll be doing it for everybody, including people you don't agree with, like perhaps "people who don't want abortions happening"
Nope, there are tons of ways to catch this crime and punish the criminals. Let's use them and enough with the self-promotional I'm-upset-so-I-have-a-right-to-break-the-rules bullshit. No amount of drama here is going to change my mind about the firm necessity of respecting private property in the abstract (meaning assuming there isn't more to the story.)