Among other things I work wood for a living, I know a little bit about wood movement and moisture content. If unstabilized wood is left in a given environment long enough, it will reach an equilibrium moisture content depending on the relative humidity and temperature of that environment. For example, at 50 degrees and 50% Rh, wood will eventually have a moisture content of 9.5%. In those conditions it won't get any drier, and if it was drier than that when you put it into those conditions, it will absorb moisture until it's at 9.5%
Green wood can dry at high humidity, but it won't get to the 8-10% we'd like to see before using it. At 90 degrees and 98% RH, wood will achieve an EMC of 26%, only about 1% drier than it will get at 50 degrees and 98% RH. But that's still fairly "dry" considering some species of lumber start out as high as 300% water by weight. Fiber saturation for most species is around 36-38% moisture, below that moisture content the wood shrinks. How much shrinkage depends on the species and grain orientation of the particular piece, and how much below the fiber saturation point the moisture content of the wood is. Given the small dimensions of a knife handle seasonal wood movement is not likely to amount to much more than a few hundredths of an inch, unless you leave the knife where it's at the full mercy of the weather. But for a table top in a house heated in the winter and not air conditioned in the summer, somewhere humid, a 2" change in width is quite possible.
Even if there's no evidence of water, many basements have a fairly high RH, hence my original comment. A drying cabinet as Stacy suggests will help if the RH of the basement is high. The heat will lower the RH of the air in the cabinet a bit as well as raising the temperature, put the thermometer/hygrometer in the cabinet and you can get an idea of the MC the wood will eventually reach.
