I agree with the others - if they start hiding bottles they ARE an alcoholic - they are so crafty that there are probably a lot more bottles hidden and being used than you know.
Sadly, however much you want to help, you can do nothing until the person recognises that they have a problem and want to do something about it. We have been through this so many times with a close relative, numerous suicide attempts (mostly half-hearted), periods in rehab, etc etc.
You can draw their attention to it, but we have found it just makes behaviour even more crafty, driving the drinking underground. Even in rehab where they were away from the world they were breathalised every day. In one hospital they used to get bottles of lemonade, tip some away, fill up with Vodka (heaven only knows where they got it from) and then use a dab of instant glue on the cap so that when the staff tested the seal, it felt as if it was intact
Sorry to be pessimistic - but it is better to be ready for this - if you do get a chance to help, it is better to do it being realistic and knowing what you are likely to be up against.