I'm still not following you. What clues did he provide?
As far as I know, no one even imagined that there was a code in the media letters until Ivins was observed throwing away "the code books" at around 1 a.m. on the morning of November 8, 2007. I imagine it even took considerable time for them to figure out WHY Ivins threw out those materials.
That act was hardly a "clue" that Ivins knew he was providing to the FBI. His home, cars, office, safe deposit box and everything else he owned was searched on November 1 and 2. Because it is COMMON for guilty persons to destroy evidence after such searches so it won't be found in the next search, the FBI watched Ivins home and, sure enough, he threw the materials into the garbage and made sure the garbage truck had hauled it away.
But the FBI agents stopped the truck when it was out of site of Ivins' home and did their search there.
I wouldn't call that "providing clues." I'd call that "making a common mistake." You might argue - with 20/20 hindsight - that he should have burned the "code books" somewhere inside his house, mixed the ashes with granola and fed it to birds in his backyard. Yeah, but he didn't know that the FBI was watching. He looked around and didn't see them. He was acting the way guilty people act when they think they are getting away with a crime.
I'm still not following you. What clues did he provide?
You're the one who wrote about the "attention-drawing features" of the message.