What do I do with mildly fermented maple syrup?

Yeah, syrup can ferment + convert sugar to alcohol. It has more of a tendency to do so if the sugar content is lower -- I tend to err on the side of overconcentrating my syrup.

You can try boiling it for a while to see if the alcohol boils off + if the flavor is OK then use it... but I'd boil down a bit more first, to make sure the sugar content is back up to standards. Either boil until the boiling point is 7 degrees F higher than the boiling point of water at your altitude, or boil until the syrup "aprons" (e.g. starts to drip in a sheet rather than discrete drops; a flat edge of a metal spatula works well), with the former being more accurate if you have a good thermometer.

If the flavor remains after boiling, then try using in recipes -- perhaps in brownies/blondies or with ice cream.

That sounds like it would work well for glazing pork or maybe smoked chicken/turkey.

There is just one thing I can think of. Maple Sugar Candy :-) The alcohol ferment will cook off some and leave behind the sugar.

It is hard to correctly answer your question, without knowing all the facts. Was/Is this Canadian Maple Syrup? Vermont Maple Syrup? Or Maple syrup from another part of the US? Vermont's regulations for the production & labelling of Maple syrup, are similar to those you find in Canada. Canada has some of the most stringent laws concerning the collection, processing, bottled and labelling for Maple Syrup in the world. If your Maple Syrup is from Vermont or Canada, I would throw it away, as it has become contaminated with something that most definitely would be toxic if consumed, and no amount of boiling it will help, it would actually concentrate the toxin even more. If your Maple Syrup is from another part of the US, (or another country all together) then you might not have 100% maple syrup &/or the concentration of the maple syrup could lean itself to fermentation, which would make a lovely mead like liqueur. In Canada they actually use Maple syrup to make liqueurs, wine, etc. However to do this they have to adjust the maple syrups sugar concentration to allow for the fermentation to take place. I would say better to be safe then sorry, and ditch it. If you do in the future decide to buy the larger container & try re-bottling it, be careful that you get your bottles extremely hot & boil the maple syrup to get it extremely hot as well, prior to pouring the maple syrup into the jars to seal. Good luck!

I think there is WAY too little water and oxygen in maple syrup for a yeast fermentation, so anything that grew in this syrup is probably bacteria or worse. Don't drink it.