I have also found that transparency maps cause this. The transparent parts of the material are not invisible to raytracing, and cause AO on nearby surfaces - see attached example, this is Rhiannon's Elsa Wylde with part of her skirts transparent.

The only way I found to get rid of it is to make the item invisible to raytracing. Trouble is, it then completely disappears in reflections.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)