My brother-in-law lost his new phone in a restaurant once. We know he had it just before entering the restroom, but not afterward. No one else went in after him, so he had to have dropped it in the toilet and flushed it, because it was nowhere to be found. Thank goodness it was his work phone and they replaced it easily enough.
As for reading material in someone else's restroom, I am at their house to visit them, not read, so I don't care if they have material provided. I think they would start banging on the door, wondering if I was ok if I disappeared for too long. I deliberately DON'T take my reader into their restroom.