Sounds like scripting the find is what is causing a problem for you.
Enter Find Mode [. ]
set field [yourTable::ReturnDate ; "<=" & get(currentDate) + 1]
set field [ Yourtable::RentalStatus ; "returned"]
set error capture [on]
Perform Find [. ]
Should I be setting this script to the return date field? That was my other issue. Since I want it to be automated, I didn't know how to define the script itself (ObjectExit, etc) and on which field.
I would set up a server schedule to run a script that includes code like this with code to send out the email messages.
That way this can happen automatically once a day.