Possible reasons for washing a car late at night?

One neighbor is a sad case. She has been to drug rehab numerous times, in the past 18 months at least 6 people have been hauled out of her house in handcuffs (parole violations, stolen cars, etc.). During the same time period at least 3 stolen vehicles her friends parked on our street were also towed away.

Randomly she or some of her friends ‘just out of jail’ (her words) will be washing (and sometimes vaccuming) either her car or ‘their’ car after dark (and sometimes at 11:30 pm). She has washed her car in the dark twice within the last 10 days. None of them have jobs, so it’s not like after dark is the only time they have to wash a car. Also, our garages are in the back, the only way I’ve seen this (or other residents) is while coming home or emptying trash.

Any idea why people would wash cars after dark?