A failed airport security test…

… ended up with a Slovak man unknowingly carrying hidden explosives in his luggage on a flight to Dublin, Slovakian officials admitted Wednesday.
While the Slovaks blamed the incident on “a silly and unprofessional mistake,” Irish officials and security experts said it was foolish for them to hide actual bomb parts in the luggage of innocent passengers under any circumstances.

Advice from a pilot;

Some things to look for: groups or pairs of men, a passenger talking to themselves, speaking Arabic, watching crewmembers (this is different than looking), staring at the cockpit door, long stays or multiple trips to the lavatory, reading a book but not turning any pages, nervousness, being unusual by trying to fit in, taking pictures/videos, not making eye contact. When you are at the boarding area and on the plane if you notice a suspicious passenger, look for others. How many? If it is one or two then they could be planning on bombing the aircraft or just making observations of crew procedures. 6 or more? Then this cell’s objective would be hijacking the plane by brute force. Also remember that there are sleepers that try to blend in with the other passengers and could be very hard to notice. A website reports a well-dressed man in custody that was also a passenger on Delta Flight 253. After an incident, your entire plane might be delayed for security and they will treat everyone as suspects. Also expect the government and airline to try to cover up parts or all of an event.