I noticed this glitch this weekend.
I had played the game up to the part just before Raz helps Milla and Sasha free Lilli. I had a bunch of brains on me that I had collected and had Ford bring me back to the mainland/Camp using the Bacon.
I wanted to do some exploring and was at the lake when I saw one of the Camp kids (see girl in above screen cap) standing on the dock. The weird thing is that when I went onto that dock and approached her she faded out completely. I could move away and she would reappear but everytime I got really close to her (close enough to talk with her) she disappeared completely.
I figured out that I could see her because I had her brain on me at the lake but she would disappear because she hadn't been re-brained yet by Ford.
So I went down to Ford's Sanctuary, turned in all the brains I had, and went back to the lake to find the girl kissing the kid wearing the sailor hat. Of course, she no longer faded out since she had been re-brained.