Pooling 'just' around the drain grate..as in on the metal surrounding any screen has happened to me as the shower grate is lower than the lip it sits in..and the grate is domed a little from body weight on it. It can stretch has no where to go and the edges retain water...that leaves a scummy residue after it dries. Siliconing around the edge of a bad grate drain is not a solution as the silicon stains as well. Refitting the grate may help.