LAS is offsite consolidated facility located South of the Airport near 215 freeway. It's here:
http://goo.gl/MKjco
Return is drive into facility, normally is 2-3 greeters at peak times to do instant returns, NEVER been asked for ID/etc.. so if someone else is doing the return no issues. Just have a fuel receipt if returned full and it had low miles (e.g. isn't a noticeable use of fuel)
Enter the common facility to get a shuttle
Shuttles to Terminals 1 and 3 run separate. 6-8 minute ride to terminals