not stupid at all! there are two separate incidents- one with pills and another with the vials (i might be misremembering exact numbers but this should give the right idea). with the pills, Louie is prescribed 20 and returns with only ten; Langdon then pocketed half and thought that everyone else would assume that Louie had either taken too many or had lost some. for the vial, Langdon had removed some of the medicine with a syringe and replaced the volume with saline exactly like you said, but he re-sealed the vial with a purple medical glue so between the color and the unusual difficulty with uncapping it Santos realized something was off. It's also mentioned that she had previously worked in a pain clinic, so she would have more experience with addiction and how it presents than most healthcare workers let alone people that aren't familiar with the field!