It’s possible that the program has crashed. I would recommend using screen instead of nohup. You can install it by running

sudo apt-get install screen

Then, you can start a session by running screen. This will launch a new bash process that you can run python patrol.py in. First, enable logging in order to identify the issue by pressing Ctrl-a, H. Run your program, and press Ctrl-a, d to detach from the session, which will keep it running in the background. You can now disconnect from your Droplet and the screen session will keep running. Keep an eye on the log file and see if it has any information if the process quits again.