Came across the linked article, which walked through the process of hiding photos on your iPhone.

• Open the Photos app.
• Tap on the photo or video you want to hide.
• Tap the Share arrow.
• Scroll until you see the Hide option.
• Tap Hide to hide the photo from view.

And to find them, just head over to Albums and tap Hidden. And if you really want to get tricky, head over to Settings > Photos and disable the Hidden Album switch.

But all that trickery won’t fool anyone who knows about this setting. Trivial to find and undo, revealing all your hidden photos.

How about adding a Face ID verification to enable the Hidden Album switch? At the very least, that prevents someone who’s got hold of your phone from snooping. Just a thought.