Could you perhaps have turned on Lift to wake for you S23 Ultra? Use the Lift to wake feature on your Galaxy phone (samsung.com)
Please check your settings if this feature is enabled, it may be causing it.
Please try Rene's suggestion and see if turning of Edge lighting will fix this issue for you!
Just search for "Edge lighting" on your app drawer :)
Thanks for reaching out to us here in order to find out what's behind your Samsung Galaxy S23 Ultra's sudden light up even you are not touching it and no notification.
Before you try out the recommendations below, be sure to check if your device's software and related apps are updated to the latest version. To update your mobile device's software, follow these steps:
In addition to TeamGalaxyDD's reply, you may check if has something to do with the Edge lighting and or third-party app you have installed.
Edge lighting allows you to use the entire edge of your display as a notification light, giving you the freedom to easily see when you have a new notification, without having to open your phone. With Edge lighting you can change the style and color of the notification as well as controlling which apps use the feature.
Change which app notifications use Edge Lighting.
Edge lighting allows you to control which app notifications will appear as normal and which will light up the edge of your screen. You can choose to have all apps use Edge lighting or only use the feature for certain apps.
Step 1. Open the Edge lighting settings.
Step 2. Tap Manage notifications.
Step 3. Tap All available apps to use Edge lighting for all your app notifications.
Step 4. If you would prefer to control which apps use Edge lighting, tap each app you want to use Edge lighting for.
For more assistance and guidance, you may contact us via our online Samsung Customer Care support channels below.