According to the National Counterterrorism Center, a popular hobby that appeals to many photographers, artists, and other hobbyists may be aiding terrorists.
In a recently released unclassified report from November 2012, the National Counter-Terrorism center suggested that urban exploration, a popular hobby that involves taking pictures or videos and creating sketches and diagrams of run down urban areas, could be helpful to terrorists.
The report defines urban explorers as "hobbyists who seek illicit access to transportation and industrial facilities in urban areas" and goes on to say that the pictures, video footage, and diagrams they often release online could be used by terrorists who are interested in scoping out a new potential target.
According to the report, this hobby can give terrorists more information on security vulnerabilities and possible access locations, making it easier for them to plan an attack. The NCTC also says that any "suspicious" urban exploration activity should be reported to the FBI or a local fusion center.
An example of urban exploration can be seen in this video and numerous others like it across the internet.
The National Counterterrorism Center's official website can be found here.