Black screen padlock / "internal recovery" ???

Hi, Recently my Macbook Pro has shown a black screen with a padlock on startup. Having had failed hard drives & logic boards giving black pages in the past, I was pretty unnerved.

However, I pressed & hole the power button to make it shut down, then started it up again normally.

Searching, I see this could be the problem:

"if the internal recovery drive is corrupt, your Mac automatically switches to the Internet Recovery mode to reinstall macOS from the Apple server..."

I ran disk utility first aid a few times.

That didn't help as the next 2 mornings on startup, the same thing happened = first the black padlock, force shutdown, second try then starts normally.

Other than Disk Utility, I don't quite know what to do. Is there some other way I can try and fix the 'corruption?' (while the laptop's functioning ok.)

It's a 2017 Macbook pro - a bit ancient in Apple-years - but still been perfectly good until now.

Any tips would be gratefully received! 🙂🙏