What’s the brand and model of the contact sensor? Sometimes it is something as simple as a school bus that goes by or warming temperatures.
First, though, make sure that it is in fact the contact sensor that’s causing the issue.
You’ve probably already done this, but check the individual event logs for the sensor to make sure that it was in fact triggered.
In particular, check to make sure that you don’t have a once a day battery check or device wellness check that might be reporting a poll response from that sensor as an event.
After that, check the notifications or the full logs to make sure that it is that sensor that is triggering the intrusion.
If it was me, I think the first thing I would do besides checking the logs is just take the battery out of that sensor. Then if you still get an intrusion alert the next day, you know it’s not that sensor. If you don’t get an intrusion the next day, it probably isn’t that sensor, and then you can start tracking that down.