I'm pretty certain they were referring to an earlier post that kind of implied that because Carlee was wearing the applewatch in that photo, that she must have been wearing it that night (it was found in her purse - ergo a potential 'wilful' disappearance). Whoever originally put that photo out there though only states that this was the last outfit she was seen in and not that the photo is of her the day she disappeared - that fact really needs to be made clear for all the speculators out there.
About the watch and all the speculating going on as to what it infers; the fact that it was found in her prurse really means nothing until we learn what Carlee's habit was. Some people wear them all the time, some not, some only while training, some not, some take them off as soon as they die, some take them off only when they go to charge them. In this case, we simply do not know what Carlee's habit is and thus we cannot transfer "what I'd do" onto her nor infer any kind of scenario based upon this.