Workman throws things into uncertainty. Do *not* mention anything to the suspects or you will tip your hand. Not to say this is a game, certainly. But if you want to try to suss out who it is, since you say either scenario is unacceptable, then I would bait them. Sounds bad, I know, but I would really want to know. If it's BIL, he's likely to lie when asked. If it's the nanny, she's likely to lie when asked. If either is accused wrongly, you've got a problem.
Put another bottle down there already opened and half gone. Create an etch in the glass to measure the level. Put a cam in there and let it go. Or watch like a hawk when the test is occurring. Thats' what I'd do. Sadly.
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