Will runs to put a robe on after Leo walks in on him reading in the nude, and at first the robe is tied so that one of the straps hangs much longer than the other. But then suddenly mid-scene the straps are both hanging at equal length.
When Leo arrives back from Africa, he walks in on Will sat on the sofa reading in the nude. However when Will jumps off the sofa quickly (headed to the bathroom) you get a clear glimpse of the modesty underwear being worn.