Hell, it makes me angry when I discover utility locators have been on my property without permission or notice.
@Cowboy is correct in that this time turned out better than expected, which is a sad state in and of itself, in that they went away without destroying or killing anything. That being said, I don't think it is acceptable for anyone, including under the claim of law enforcement, to trespass on someone's property in this manner. Which brings me to the (legal) question of, what would they have done had the property been fenced, gated, or even marked No Trespassing, indicating that the owner is expressly exerting their claim of privacy, meaning the cops can't claim they were in some sort of public-ish space.