Unsealed jar of preserves sealed itself in refrigerator over night. Safe to remove from fridge?

I have always read not to test the seal until the jar was completely cool. That said, I would keep it in the fridge, and use that jar first. Jams and jellies tend not to be very volatile. My grandmothers would just seal the jars with a layer of paraffin .

While everyone is coming down on the side of caution, to be very clear just because something sealed does not mean it is sterile. Sealing works after processing because any bacteria have been killed by the heat. So something that sealed in the fridge might be teaming with microbs and may not belong on the pantry shelf.

Air contracts as it cools, so when you put the jar in the fridge, it must have been sealed enough to create a partial vacuum when the air trapped under the lid contracted, pulling the center down. It will most likely pop back up when it warms to room temp and the air re-expands, but this doesn't mean your jar isn't sealed. Was the jam not very hot when you put it in the jar? Hot air contracting is why jam jar lids ever pop down when they seal -- when you put hot jam in the jar and close it, if the lid forms a seal, the lid will suck down as the jam and air cools.

Not sure why that happens, but I'd keep that one in the fridge, just to be safe.