We had the same problem, so we marked all of our wheelchairs with "Volunteer Department" in large bright letters, which helped. This past year, we started investing in Staxi chairs - they don't disappear, they can fold up and so they don't get stored in storage rooms on units and can't be put in trunks of cars. It's amazing - they even make it back to our Information Desks because no units want them. They are very easy to use and can hold small and very large people.