If an ordinary citizen cannot do it, I don't think police should ordinarily be permitted to do it; there should be a review. Given this lasted for months, there clearly wasn't immenent danger.
People have a right to be secure in their houses under the 4th amendment. People have a reasonable expectation that others will not install cameras on property they do not own for the purpose of surveiling a house the other does not own. Secretly installing a camera seems like a violation of that reasonable expectation, thus making it a search.