A fascinating, if weird article on IS use of scouts to conduct recce in Europe, and originated with a Polish investigation into a suspects use of Facebook which can be used to discover their wanderings:
They look like students taking a gap year. Traveling through Europe to get a sense of its culture, to party, and to catch a holiday romance. But they are in prison now as suspected scouts of the Islamic State. The Belgian daily newspaper Het Laatste Nieuws delved deep into their Facebook accounts, and unearthed an alarming strategy.

The Belgian newspaper article is behind a wall and language barrier; the Polish article has no option for English. The link is to a Belgian blogger on terrorism in Belgium, not seen before.

A sample passage:
The journey of Taleb reportedly was nothing less than a reconnaissance operation. Commissioned by Abaaoud, who was already planning several Islamic State attacks in Europe by then. For that, he used several scouts. Most of them were traveling through Europe under the disguise of Syrian refugees, reporting back to Abaaoud about their means of transportation, places to stay, and most important: how they passed borders and other controls.