I lost my sense of smell when I was pregnant for the first time--this was in the early 70's, and I've only just begun to have some small sense of smell. I am not sure if this is what the Mom suffers from, but Vodka is the least "odorous" of the hard alcohols, and if she reached for a water bottle and was expecting water, she may really not have noticed. I know I can't smell vodka or rum. I do smell whiskey, and some wines. If this mom had a diminished sense of smell , and remembering that she may not have known of her boyfriend's replacing the water in the water bottle with Vodka, she may truly not have noticed.
As an aside, my oldest son works for a construction company and heads a crew of workers-----he tells me that his company has BANNED water bottles on the job sites because it is COMMON for some of these workers to substitute vodka for water.........the company provides a large water dispenser. Even then, my son has found beer hidden in port-o-lets (even under the seat, GAG) , discovered whiskey in coke bottles, etc.
As far as for not making continuous eye contact, she did lift her eyes occaisionally. She looked more ashamed and upset than guilty to me.