Anyway, the gist of his point was that there's a tradeoff between evidence in plain view, and invasive security checking.

Police are allowed to use evidence of other wrongdoings while investigating a completely different matter, and switch their investigation priorities, as it were. For example, a car might be stopped for some minor infraction - a broken taillight perhaps - but if the officer sees (say) drugs in the back seat, they can change their priorities. Evidence in plain view is the general scope.

The other side is that security checkpoints at airports go far further looking into things than would meet an "in plain view" test. The problem with being able to switch investigation priority in this circumstance is that it evades protections against unreasonable search.

His suggestion was that officers could do one or the other, but not both. Security screening should be strictly limited to physical security, and not seek evidence of other wrongdoings.

There is some evidence in any case that judiciary are starting to push back here, even in cases where actual wrongdoing is discovered by security searches:

http://online.wsj.com/article/SB1000142405297020455680457426...

> A federal judge in June threw out seizure of three fake passports from a traveler, saying that TSA screeners violated his Fourth Amendment rights against unreasonable search and seizure.