This is a tricky question, and the response will vary greatly by the facts of your incident. Some employees, as you likely know, are "on call" 24/7, such as police officers, EMTs, health care providers, and emergency response providers. With others, determining whether they are still "on the clock" or not will vary.
Feel free to contact me with specifics, or look at some of my blog entries at www.gaworkerscomplaw.com.
If they are injured while still on property even after hours, they are likely to be entitled to workers comp benefits. If they are hurt off premises while traveling home, in most instances, this will not be a liability concern to you. There are lots of different rules for on call workers and the like so it sounds like you need to talk to a lawyer. We offer free consultations.
There are cases that allow for coverage in these situations. It primarily depends on what you were doing after hours. If you were injured while doing something that was job related you should be covered. Some more information is needed as merely knowing that you were on the premises after hours is not sufficient to answer your question. I would recommend that you consult a WC attorney to answer your question more specifically.