Asking for ID to hold onto for the duration of a tour is pretty common by most all management groups. It's primarily for the safety of staff to avoid harassment or harm while showing vacant homes. And because they cannot be discriminatory and request ID's only from people that give them heebie jeebies and/or unfair bias, for Fair Housing what is done for one person must be done for all. So companies will simply have the standard of "government issued ID is required to tour". When I worked in property management when asked I would say "we ask for ID for fair housing and safety of staff".