People see through transformation techniques when there is some flaw in the transformation, not because high level people just automatically see through them.

They see through them because they are a puddle when it hasn't rained, or because they are a person and they missed a scar.

But changing into shrub in a place full of shrubs shouldn't pose a problem unless the people you're trying to fool have been there before and notice a shrub that wasn't there before.

I thought transforming into the door they blew down was an especially smart move.