Five Transportation Security Administration employees have been placed on administrative leave since it was discovered that sensitive guidelines about airport passenger screening were posted on the Internet. The move was disclosed as senators questioned administration officials yesterday about the second embarrassing security flap at the Homeland Security Department in as many weeks. Assistant Homeland Security Secretary David Heyman told senators yesterday that a full investigation into the Internet security lapse is under way, and that the TSA employees have been taken off duty pending results of that probe.