Yup, probable cause, no question about it.

FORBES: Government to Argue Detention Based on Arabic Flashcards Was Justified

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

FORBES: Why You Should Think Very Seriously Before Expressing Political Views at Airport Security

The Home Office's extradition review, led by former Court of Appeal judge Sir Scott Baker, argued there was no real difference between the US tests of "probable cause" and the introduction of "reasonable suspicion".

BBC: US-UK extradition: The law explained

No official reason is given for this, but one probable cause is that hiring female staff would put a lot of men out of work - not a popular move in a country where 13% of men are unemployed.

BBC: Saudi lingerie trade in a twist

An inquiry in 2008 concluded a nuclear submarine snagging the boat's trawl was the "highly probable cause" of the sinking, but the judges recommended the investigation be wound up, with no guilty party traced.

BBC: France probes US submarine link to fishing boat sinking