there’s not a great way to find the photo pages for orphaned photos other than by brute force, which means that if you know the id of the photo page for a photo that was added before or after, the orphaned photo’s photo page id should be somewhat larger or smaller, respectively. so you could just increment the photo page id until you find the one you’re looking for.
that said, once you find it, there’s not a great way to reattach it to your observation. also, the orphaned photo will eventually get deleted from the system. (i forget exactly what the schedule for deletion is. maybe a week or so.) also, if the only observation associated with the photo gets deleted, that may cause the photo to be deleted right away, not orphaned, but i’m not sure.
i asked for a feature to find these orphaned photos back in the day, but it hasn’t gone anywhere: https://forum.inaturalist.org/t/find-your-own-orphaned-photos-uploaded-to-inaturalist/6610
if you just want the photo file, you might be able to get that file location if the observation was pushed to GBIF but hasn’t been updated sans that photo yet. or if the last AWS open data set recorded the location of that file, you might be able to get the photo location from the AWS open data set’s photo metadata file. you could also try the brute force approach.