Dulles Logistics with Overnight Layover

You might want to check this:
https://www.sleepinginairports.net/guides/washington-dulles-airport-guide.htm

What airline? If you go through the main immigration area, you will exit outside of the secure area. The airport is open all night, but I am not sure that you will be permitted to go through security if you have a ticket for the next day.

United has an immigration area in the United terminal but that only applies for United passengers with United connecting flights. It sounds as though this doesn’t apply to you.

Thanks!
Will be arriving via Ethiopian Air (8pm) and leaving on Delta (6am). I was hoping to be able to stay on the "security" side of the airport, not have to exit and re-enter since I don't think I would be able to reenter the night before my flight leaves.

The airport website says that there is after hours security screening next to the TSA Pre-check lane.
https://www.flydulles.com/travel-information/security-information

With Ethiopian, you will go through immigration in the main terminal and exit to the non-secure area. You will then need to go up the escalator to get to the ticketing area and go back through security. Again, I have no idea if there would be any issue going through with a boarding pass for the following day. However, it is worth trying as the area pre-security won’t be a pleasant place to stay up all night. The area post-security won’t be great either, but it would be better.

Thank you for the link and info.

I will give it a try and see if I can get back through security overnight with my boarding pass. It will also motivate me to make sure I fit everything in a carry on size so I don't have to check luggage.

The TSA generally is unlikely to let you enter the secure area with a boarding pass for the next day. Sorry but that’s just reality of the situation.

These are not connecting flights. These are two completely separate flights and as a result, you may be stuck on the wrong side of security. Neither one of them are great places to spend the night.

Thanks. Might just have to wait for early in the AM to get through then. Challenging when flights arrive late and no other flights heading out until morning.