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Perhaps this is different for various kinds of butter and of course the temperature that is applied to it...
Most (USA-generated, I assume) butter is ~18% water. Without approaching the steaming temperature point of pure water, does simply melting butter and then allowing it to return to its initial saturated (solid) state affect the water concentration?
In case I'm seeing this from the wrong perspective, perhaps this should be seen as a matter of distribution rather than concentration.
The proportion of water will not change significantly unless you leave it hot for a while (evaporation will occur even below boiling temperature); the only substance that will evaporate in any meaningful quantity is the water so you can check if this has happened by comparing the original mass to the mass after melting.
However, the changes to the butter will be more significant than merely redistribution: see the information at this answer for more detail.
