Lots of plain view jurisprudence relies on the fact that if it can be observed by random people -- not just by law enforcement -- then there's no Fourth Amendment issue. If airplanes can pass over someone's land, surely police helicopters can do the same thing without undoing expectations of privacy.
Some of this judicial thought process has been altered by persistent surveillance from law enforcement cameras -- ones that don't just observe, but also record and provide officers with searchable footage of residences investigators are interested in. Then there's the incidental aspect. If a cop enters a home to perform community caretaking functions and spots contraband, this is legal as it's not the point of the cop's entry. If the cop is there solely to look for contraband, a warrant and probable cause is needed.
But a brief overflight generally isn't a Constitutional issue, no matter how high a fence those under investigation have constructed. A flyover isn't persistent or invasive surveillance. But tech advances have altered how flyovers by government agencies are conducted. In this case, via FourthAmendment.com, the Michigan Court of Appeals has found in favor of a defendant who moved to suppress evidence gathered by the city with its drone.
And this is still very much law enforcement activity, even if it wasn't related to the sort of crime we normally associate with constitutional violations.