Is there any reason to store maple syrup in the refrigerator? It tastes better when it's a little warmer.
I'll assume that you're talking about pure maple syrup in a glass container; if it's that adulterated pancake syrup then it's probably riddled with preservatives, so any advice here doesn't apply.
Pure maple syrup can and will grow mold on the surface if left in a cupboard. There are several reports of this happening, and although several of those people say that it's OK to simply strain the mold and re-boil the maple syrup, (a) I wouldn't chance it, and (b) that process is hardly any more convenient than simply taking it out of the refrigerator a half-hour earlier and letting it come up to room temperature.
Maple syrup should be stored in the refrigerator. It doesn't have to be, and it will probably take at least a year for it to grow any mold if left in the pantry. But it will last longer in the refrigerator; I've seen refrigerated jars 3+ years old without any mold.
Items like maple syrup, honey and jam have a very high level of sugar in them. This causes a high osmotic potential which is what preserves them.
The problem when you refrigerate these types of foods is that water can condense on the surface. This will dissolve some of the sugar underneath it and lower the concentration, and therefore the osmotic potential, around it. This can be enough to allow microorganisms to grow.
As long as you are using the maple syrup up within a sensible amount of time it will not require refrigeration.
Above link states that the FREEZER is best for long term storage of pure maple syrup. It is not supposed to be able to freeze solid since it is only 33-35% water.
Maple can grow mold, but that doesn't mean it's trash. Pour it through a strainer into a pot to remove the chunks, then heat it to 180 degrees for 5-10 minutes (do not boil!!). Scrub and rinse the container and pour the syrup back in. This will kill anything lingering in your syrup, and also the bottle, without hopefully affecting its quality. If the syrup once cooled smells or tastes off, then its trash (not unhealthy per se, as anything icky is now dead, but who wants to eat offtasting syrup)