bullwinkle
Pattern Altitude
I don't know of any legal prohibition against, and I have no concern over, an LEO who sees some flying that concerns him/her walking up to the pilot after the landing and saying "Hello, welcome to Friendlytown Airport," and then seeing if the pilot reeks of alcohol or otherwise then acts in a manner indicating being under the influence -- something which any police officer is well-trained to do. And since violation of 14 CFR 91.17 is an administrative rather than criminal law manner, the normal requirements for probable cause do not apply to any ensuing FAA action.
Yet another reason why, when approached by "friendly" law enforcement personnel, the best response is "I'm sorry, I don't speak to police."