For example, uncommon spices they frequently purchase, unusual soaps which appear on the Von's supermarket account, strange use of phone cards, mini-storage units at the same address, people coming together in cities with no record of their habitation. This is just a fingernail clipping of the landfill of information "brilliant data mining" sifts and collates, looking for connections adding up to a terror cell.
No one set of factoids is damning, a "smoking gun." This is where folks are misunderstanding things like the FBI looking at library records. The library records are not useful for one person and one book, but as part of the vast constellation of data points which in aggregate make up the picture of a terror cell.
Little or none of the data mined information is used in a conventional evidenciary manner. The goal is to ID possible terror cells, and then apply old fashioned surveillance techniques to find the confirmational evidence, and go if for the arrest, or for the kill.
Which makes the dismantling of HUMAN intelligence under the Clinton regime all that more despicable.