EVs often operate more like laptops than cars, in that when you turn them off they don't really shut down - at least not for a while. When you close your laptop, it just goes into a "sleep" mode, otherwise known as "Standby". That laptop is still working in the background doing things like receiving emails, indexing your photos, and remaining ready to boot up quickly when you open it again. Only after being asleep for a while will it truly shut down, or go into what's referred to as a "deep sleep".
Many EVs operate in a similar manner. When you arrive at your destination you can turn the car "off", but it may not shut down for quite a while. The car, like your laptop, is still working in the background on things like managing the temperature of your battery or downloading software updates. It's also staying ready and monitoring for when you might ask it something - like each time you open your car's app on your mobile phone to check your range or lock/unlock your doors, the car will wake up to return the info that you've requested over a data connection.
If you're not plugged in, it's easy to imagine how these tasks could use up some of your available battery, leaving you with less range than when you initially parked.
The amount of range that you may lose depends on how long the car is doing these tasks, and what sort of tasks they are. For most cars, Vampire Drain is insignificant. You might lose a few miles per day, on average. There are some features, however - like some security features that actively monitor surroundings with cameras - that draw enough power to be noticeable.
This also means that if you need to park your car for extended periods without plugging in, like at the airport for weeks while on vacation, you may want to turn many of these background features off.
If you do need to park your car for long periods, here are a few features you may consider turning off (if your car is equipped with them, and if the car allows you to turn them off):
In addition to these features, you may want to reduce the number of times you open your car's app on your phone. Each time may "wake" your car and use more energy.
Bottom line, if your car has a lot of connected features and you plan on using them (let's face it, most are fun and useful) than you'll want to factor some vampire drain when you consider the size of the battery that you need.