If you don't have a catflap, you can install a cat doorbell instead. It will let you know when your cat wants to go back inside. I use mine as a dinner bell for one of my TNR cats.
The wireless cat doorbell consist of a sensor and a receiver (bell) and is easy to install. You place the motion sensor above the spot where the cat usually sits when it's waiting for you to open the door. The cat will walk under the motion sensor and this will activate the receiver. The receiver will either flash, ring or both. You can adjust the volume as needed.
A TNR rings the bell
I have two indoor cats and thus don't need a cat door bell for it's intended purpose. But I also have a big colony of community cats that I TNR and feed. One of my TNR cats don't live in the colony and therefore has her own feeding station. While the colony has acces to food all the time, I have to serve Little Poul her dinner every night in order to make sure she gets fed. Other cats plus the neighbour's dog will come by her feeding station, so I can't just leave food out for her. It may sound less than ideal, but don't worry - she is in fact very spoiled! I feed her the best stuff, she gets kitty milk and treats.
Lille Poul's tree
Little Poul has a routine. She comes by every night and waits for me to bring her dinner. She always waits for me in a tree right by her shelter, sitting on the same branch every day. The time she shows up varies depending on the season and the weather. Sometimes we miss each other and I have to put the food out and hope that she will come by soon. In order not to miss her, I got the idea of getting her a cat doorbell. That way I will know when she is ready, rather than going out too soon or too late.
Ready for dinner
I put the motion sensor in her tree, right above the branch she usually sit on. When she jumps up in the tree at night, the bell will go off in my appartment. The bell is placed in a window, so the two devices can reach each other. When the bell goes off I know she is waiting for me and I will go serve her dinner. It works like a charm!
Little Poul is about 4 years old and has lived outside her entire life. I have been feeding her since the winter of 2016 and she is the one of all the community cats that trust me the most. She is not tame though. But most days she trusts me enough to let me stand next to her while she eats. And she loves the food I serve! You can read more about being a caretaker for community cats here.