I assume your decanter is transparent?

Lots of people assume Jim Beam whiskey got its name from some fellow names James Beam. But actually it’s a reference to its dual nature: the beverage can exist as both a liquid and an electromagnetic wave.

This is why Jim Beam is so cheap. They want to get it off the shelves before it disappears via Hawking radiation.

Your whiskey didn’t actually disappear. It changed.

Sorry to be the bearer of bad news – you subconsciously prefer the Jim Beam and are sleep drinking it at night.

Occam's Razor: prefer the answer with the least assumptions. The simplest answer is that the stopper leaks.

You've only got a very small sample size and it's possible that the stopper is slightly asymmetric and fits well one way and no other.

The easiest way to unreliabily detect this is just to rotate the stopper in the neck and see if it sits in one place sightly differently than other positions.

Also, if you're more frequently drinking your preferred bourbon, you'll have a harder time noticing any evaporation.

You got kids?

My suspicion is that your stopper isn't sealing well, possibly from wear or just odd positioning. If you feel like ruining what's left of that Jim beam in the decanter you can run a little experiment. Clear the condensation out of the decanter and mark the current level. Leave it for a week and see if the level dropped to establish a baseline. After marking the change, coat the stopper in Vaseline and leave it for another week. See if that helped or not. If it helped then you've got a leak

You just have a lot of angels that want their Angel Share

Not what you're asking for, but unless you know your decanter is made from lead free crystal (most aren't), don't store booze in it for any amount of time. You should be safe to use it to serve a spirit in, but lead leaches into the spirit much quicker than you'd realise

Is your decanter over something warm? A TV? Cable box? Radiator? Heater register?

Something that will cause the likelihood of evaporation to go up?

If it’s lead crystal then don’t store alcohol in your decanter. It will slowly leach out over time. Crystal is meant to be used then stored away.

Different alcohol sources evaporate at different rates.

An experiment:

• Zero out a scale with a glass
• Put 4oz of Beam in the glass
• Zero out a scale with another glass
• Put 4oz of the other bourbon in that glass
• Record total weight of each glass
• Wait 24 hours
• Weigh them again to see change
• Pour fresh glasses
• Try all 4 back to back to compare alcohol taste

My guess will be that unless you have a coffee or herb scale, you probably won't see a measurable difference, but you'll be able to taste more of a difference in the Beam.

How many kids do you have.

Do you live alone? I mean I've heard lots of stories where kids, spouses, visitors ... help themselves. And some booze just vanishes, or gets replaced.

your roommates are drinking it.

Condensation means there is more water evaporating, not alcohol.

Keep it in a cooler place and this will reduce the evaporation rate.