Signs of leglimency can be detected, so if Quirrell is leglimizing Harry, he may be risking getting caught.

The fact that Harry has an occlumency teacher who uses leglimency on him in an instructional setting might serve to disguise other people using leglimency on him though. We don't know what kind of evidence checking for leglimency can provide about recency and whether the mind has been accessed by different people.

However, I doubt that Quirrell has been using leglimency on Harry, because there's a point where you're really just giving the antagonist too great an advantage, and if Quirrell has been accessing the contents of Harry's mind undetected, I would have an incredibly hard time buying the protagonists not being completely fucked (tvtropes link.)