Looks like a Tomato Hornworm to me. Manduca quinquemaculata

Petunias are in the same family as tomatoes, Solanaceae. I imagine that's the attraction of your balcony.

Thanks. I think it is tobacco hornworm. It was on a well past its prime nicotiana alata, trying to eat the stem. I have too many nicotianas, originating from my compost than I can find use for. I also have hundreds of wave petunias that I sowed pretty late and some of which are just starting to bloom. I also have physialis pruvian and bell peppers. I flicked it off the balcony but it may have landed on the edge rather than falling down. I will keep an eye out for it. May be it can have the spare volunteer nicotianas.

Oops. I need to concentrate. I misidentified the flowers.😱 But any Solanaceae would attract them.