They aren't going to call the cops if you bring back the car back a few hours later than you stated on the reservation.
I started the thread because I actually had this situation about 5 years ago with Alamo. I took the car back the next day, still within the 48 hour window, but because it was the next calendar day, I was charged a late fee (about $40 if I recall).
Maybe Alamo is less lenient than others. I remember a couple of years ago I was getting into MCO close to midnight so I had a car rented for 12:30am the next day. My flight got in early about 11:15pm and they made me wait at the counter until it was 12:01am so it was in the same day as the reservation.
Go figure.