Ξαφνιάστηκα όταν σας είδα φιλώντας.

English Translation

I was surprised when I saw you guys kissing.

This sentence was corrected by a native speaker on Tatoeba to: Ξαφνιάστηκα, όταν σας είδα να φιλιέστε. I think this is much more idiomatic.