Actually it depends, some don't. There are some that just update them and if the computer cannot be contacted, it doens't change it (like it's off) and the record stays there, others remove them if they can't contact them. Some also link in with the DHCP server and update that way. The best thing todo if the waiting game achieves nothing is ask your admin to do it for you, it's not a hard task (Windows' Servers anyway) and it takes like 2 seconds.