Are you sure? At my company we have very flexible time, and due to city traffic, many people come in around 11-noon, meaning they choose to leave around 7-9. This is much more convenient for them because of the traffic, and sometimes it better matches a spouse's schedule, or allows them to do something in the mornings they wouldn't normally be able to do. It may have been an indicator of a really good place to work. (Which isn't to say you're wrong. It very well may have been a warning sign.)
Although it's my own conjecture, it's based on numerous other signals I got from them during the interview process. As an example, any team member A can point out that any other team member B isn't pulling their weight and shouldn't be working there. That initiates a discussion about whether B should be fired.
1. A homework assignment where you have to write some code for given requirements, use a specific design pattern they require, write tests and document.
2. Have a 1 hour phone call to discuss your assignment.
3. Have a face-to-face meeting with some of their devs to discuss behavioral stuff (what motivates you, what you want out of work, how you give and take feedback).
4. A full day of technical interviews.
I did the first three steps and I thought I was done, because they never bothered to describe the whole process clearly. Then they contacted me to schedule a full day interview round. Since I didn't know about that beforehand, I had used up all of my PTO on interviews with other companies. I explained that I couldn't do a full day because of my work and they offered to do two consecutive half-day interview rounds from 4 pm to 8 pm.
Now, let that sink in: they were willing to make their devs stay and interview me until 8 pm, two days in a row. That's when I decided to thank them for the opportunity and politely bail.