OK, one person in our group found this useful thing: Monte Cook ruled this ability
doesn't cover scent before level 11th.
Trackless Step (Ex): Starting at 3rd level, a druid leaves no trail in natural surroundings and cannot be tracked. She may choose to leave a trail if so desired. The druid still leaves a scent trail, so those who are capable of tracking by scent may continue to do so. At 11th level the druid effectively has no scent trail while in natural environments; her body is fully in tune with her surroundings.
This makes it impossible to track, locate, or detect the druid via the Scent ability while in a natural setting. ( Monte Cook Wiki:
http://ironheroeshouserules.pbwiki.....php?wiki=DRUID )
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Also I thought that the Trackless Step ability is EX, and not a SU or a SP ability. So he can't remove his scent
just by being extraordinary knowledgeable in a forest.
Case closed for me

Thank you all for your input.
Joël