So just to add a few things based on my experience living on Oahu for almost 3 years.. Nothing is stated as fact here just my thoughts about the area and possible scenarios.
Schofield is more centrally located on the island. It is land locked, so not near the ocean. There is a fence that goes around the outer perimeter of the main base where the housing areas are and where the servicemembers typically work day to day, which makes it necessary to enter though one of the gates. They are all manned gates so all cars or pedestrians entering must show a military ID or they must enter through the gate where the visitor center is and they get a pass. The exception to this would be if a guest is entering with someone that has a military ID. Typically guests can just ride along in the car and unless there is a heightened threat level, no pass is needed for them. You as the ID card holder are vouching for the guest(s) while they are on the installation. Some based require a pass for anyone entering after a certain time (where I am now, it's 9pm).
People exiting the base do not need to show an ID or stop. They can drive off or walk off the base. I do believe all ACPs (access control points) have video cameras so this would be noted both entering and exiting.
There are also training areas on/near Schofield as well as hiking trails that start on the base and leave the base through the mountains the base backs up to. It is possible to hike out this way OR to hike in. It is a VERY difficult trail if someone wanted to do this, it would not be something the average person would/could do. There is a shorter trail that many do hike, but to go past that point out to the larger trail area, would require someone a bit more experienced. So I guess it's possible she could walk that trail and leave the base without being seen, but I'd say it's very unlikely due to the terrain. I'll link the all trails map of the area so you can look. Kolekole is the shorter trail and when it ends the more dangerous parts begin.. typically most hike out and back on that shorter/safer trail. More experienced hikers might go past that and do the harder parts of the trails.
Edited to add: if she walked off at night, I doubt she would have went to this trail. This is open during the day and again it makes not sense for a 6 month pregnant girl to decide to go hike this trail at night, but I wanted to include this in my post because it is a possible way to get in/out if someone really wanted to and knew how.
If she walked off the base, I feel she would be seen on a camera somewhere.
If she was driven off base, I'd say it's possible she went unnoticed and unless they know what car to look for its possible she exited this way and doesn't want to be found.
The base is also lit well enough that even at night, it's not uncommon to see people walking in the housing areas. I often went for a walk once it was dark because street lights lit things enough and with it being hot during the day, I enjoyed walking once the sun went down.