I have been there twice and never saw a cop. Granted I went early (i.e., 7 am on a Sunday morning), but still. As long as you are hiding behind the buildings you would be okay. Make sure to bring gloves and a high quality flashlight, as the place is very dark due to the windows being boarded up on the front. Since they are boarded on the front, you can walk freely in the main buildings and not have to worry about someone seeing you.
I never found the morgue in the children's hospital, but the one in the adult hospital is ridiculously easy to find. Getting into the buildings can be tricky, and the place does have an eerie feel. I never went into the basement or above the second floor, sticking to lighted areas.
The tunnels in the basement are something I never saw, as every entryway I would have seen for that had stagnant water. It would have been cool to go in those but I was a scaredy-cat as the first time I went alone.
All in all, Glenn Dale was awesome and I would love to go back. I got amazing photos there.