The most likely culprit is sentry mode, since you did not give any other information. Most people who are supercharging are also either parking outside or parking somewhere else where they cant plug in and charge it.
Sentry mode uses between 15-25 miles per 24 hour usage.
The other culprit is likely the fact that you "drove 20 miles" and expected only 20 miles to go off the range meter, and that absolutely is not going to be the case, especially if you are only going short distances.
Without any other information from you, those are the two most likely situations. To answer usual follow up questions:
1. Yes, sentry mode does actually use that much. No, changing how the cameras record in sentry mode will not change that, because the drain from sentry mode is from keeping the car awake.
2. Yes, its absolutely normal for more than 20 miles to roll off the range meter when 20 actual miles is driven. No, there is nothing wrong with your battery / car when this happens, Yes, its 100% true that the car has less effective range than the range meter shows, this is due to EPA range numbers not being indicative of actual driving conditions.
3. Yes, the car is normal, and no, taking it to Tesla or talking to tesla about your car not getting its rated range is not something you should be doing.