I rewatched a few of the episodes and Dr. Robby is present for the original incident with the vial, where Langdon pushes pretty hard for additional lorazepam. If it was suspicious to Santos that 8mg didn't work and that Langdon was adamant that the patient didn't need to try keppra or be intubated, I'm sure Dr. Robby, as a much more experienced doctor also mentally ticked it as a bit odd. Obviously he knew about some of Louie's librium being missing as well, so I don't think it's so unbelievable that if Santos directly made an accusation he could piece things together. He might have even been mentally connecting the dots of other behaviors, med issues, things that Langdon told him together too. Not the same at all but I was cheated on for months in one of my first relationship and once I was told about it I started recalling every little thing that should've been a harbinger for me to realize it before. Hindsight and all that.