How did it get into the fridge at work? Did your boss put it there? If not, and someone else did while 'picking up'... he may not know that it is his (or hers) and doesn't want to take someone else's bottle.
If he/she is the person who put it there, do they routinely use that fridge.. I mean, like, DAILY? They might have just forgotten about it. You said they went out of the country for business... I would imagine that they have had a lot of important things transpire/be going on in their head, and maybe just plain forgot.
or, given that it was a "New Year's" bottle of champagne, perhaps they are waiting for another celebratory event to transpire to warrant popping the cork on it? My hubby and I drink wine/beer frequently... but we left a bottle of champagne in our fridge for a year. We were sort of waiting for an "occasion" to open it. Since it was still here from last New Year's (2009/2010) and we had family here on Xmas, we drank it then. (It was still yummy). But we don't drink champagne that frequently... so maybe your boss doesn't either and has a full fridge at home...
Try not to read too much into it. Whatever the reason, I'm sure it doesn't have anything to do with how much they value you at work.