I don't know why, this makes me a bit sad. You can sort of feel a game engine when you're playing a game. The GTA trilogy lost a lot when they did their remastering project with a different engine. Although I'm sure it can be done well. StarCraft remastered was done really well, but I believe they built on top of their existing engine.
absolutely. whenever a friend is playing anything with specific shader/specular/lighting styles I always yell "Unity!" while watching him. I'm generally dead-on accurate with those guesses. It's definetly not coincidence at this point; there is some default behavior or lighting gimmick I can generally always cue in on with regards to Unity. I think it's their style of specular lighting glow that gets painted onto way too many things by most creators.
I don't use the engine myself, so I can't tell you exactly what it is I hone in on, but I can for most titles.