It is very easy to forget that someone is searching in restricted set of machines (available, free). Currently the only source of this information is the URL.
An enhancement could be a placement of one of the strings 'all', 'available', 'free' somewhere in the webui. 'Somewhere' could be next to the 'Systems' above the search criteria.
This enhancement can prevent filing the bugs similar to https://bugzilla.redhat.com/show_bug.cgi?id=620469#c1
This was included in the fix for bug 723753 (commit 252d336e).