Well, if he's "spying" on me, then I wouldn't know he's there, so there wouldn't be anything to be turned on about.
However, I do love the teasing element of my SO watching me masturbate and seeing how long he can hold off before he pounces. I find mutual masturbation very hot, too. I also enjoy being restrained, so I can't touch his cock, while he masturbates very close to my face and/or gliding his cock along my lips. Drives me crazy when I can't touch.
So, in today's lesson, I do enjoy being the observer and observee in all kinds of masturbation fun!
If my boyfriend asks EXTRA nice, I'll touch myself for him. It does turn me on, because I know that he's getting more and more turned on, and it won't be long before he can't stop himself from jumping me.
Had a neighbor once, spy on me as I was pleasing myself on an old picnic table in the woods. It was a major turn on. And I also fantasize a lot about my master having me play just for him or for his friends..