Maybe I can throw a little reasoning into why the winter is tough on humidors...
First off, they're made from wood. Wood looses moisture and contracts significantly in the winter. That alone is enough to create minor seal issues.
Now add the difference in the surrounding RH. In the summer it's generally around 80%. In the winter with no humidification in your house (in northern climates) it'll drop to around 15%.
Now some physics...
In HVAC terms, the industry uses the word "crack" for leaks. Anywhere there's a door or window, you measure that, add it up, then use mathematical computation to add btu's to see how much heat you need to apply to that room to defeat the air coming in.
You're also dealing with vapor pressure.
Air laden with water creates a physical, quantifiable pressure. It always tries to rush to an area with a lower pressure, much like air escaping from a balloon. It's much different from heat flowing to a cold area, there's far more actual pressure involved.
So now let's look at it through numbers...
In the summer, you're looking at 80% vapor pressure trying to enter a 65% humidor. That's a force equivalent to 15%, and it's working on a length of crack that's probably at least 95% good. Very low leakage. So there's not much exchange in vapor, period. It's easy to control with minimal effort.
Now winter comes.
You have an outside vapor pressure of 15%. Inside the humidor you have a vapor pressure corresponding to 65%RH. That's a coefficient equal to 50%. Over 3 times the strength is pushing out in the winter than is pushing in in the summer.
Now, you also have a shrunken humidor. Odds are that the 95% seal has degraded to at least 75%. Odds are it's even more, and it gets worse as winter goes along and your humi continues to warp and shrink.
So now you have 50% more pressure acting on a minimum of 20% more crack.
That's a lot more pressure going through a much bigger hole.
How can I combat this kind of thing?
Seal up your humidor.
Place your desktop in a big ziplock or plastic bag.
Run a console evaporative humidifier in the room with your cigars (I run 3 in my house all winter because my skin is crazy dry and the kids get nosebleeds if I don't).
Add active humidification.
Add more beads.
Also realize that every time you open your humidor or winador or whatever, you compound the problem. In the summer your beads only have to compensate for a 15% RH differential. In the winter, 50%. That means they need to give up a lot more water to keep your cigars at the proper RH.
All this stuff is all well and good, but nothing beats a well-sealed humidor. Lots of us have the pretty ones with glass that are made from particle board and glue. They're cheap humidors. As such, we get what we paid for. With some ingenuity, caulking, weather seal and a positive latch, I can turn any cheap humidor into one that's airtight. You can, too.
Humidors don't just magically lose RH. Lots of us think the water gets eaten by cigars or disappears magically or gets pulled into a black hole, or we don't give it any thought at all.
Your answers to where it goes are right up there ^.
Every different humi has it's own quirks. My big display humi required two tubes of caulking, an Avallo EZ, and about ten pounds of beads in the bottom to keep it perfect this year. My winador hasn't required much of anything beyong a capful of distilled water. It's nice and tight.
The good news is that they can all be fixed. Michael (cigarnut) is a master at this sort of thing. His beads are second to none and he (and many others) can tell you exactly what steps you can take to plug up your leaky humidors.
If we understand one simple principle, it makes everything very clear as to what we need to do. Humidity does not get used up or disappear. It escapes. It always wants to seek equilibrium, and a higher RH exerts pressure to go to a lower pressure area. It always wants to be equal and happy. That's what Equilibrium is, and that's how ALL beads work, whether it be Kitty litter, HCM beads, RH beads, or Viper's beads. They release and adsorb (adsorb is not a typo, beads don't absorb anything, paper towels absorb) water by this same principle of equilibrium.
That same principle is at play on the inside and outside of your humidor. The ONLY way to counter that effect is to seal it up tighter than a frog's ass.
If your humidor is losing RH rapidly, you have a big leak problem. If it's gaining rapidly in the summer, you have an even bigger problem, and bigger leaks.