Assuming it's a real scenario,
when I find a tied up, naked submissive with a sign saying "please use me" anywhere in public (assuming at night in a public park or forest) then I'd ask if she's held captive by someone. Sorry to be that party pooper but also, I'll be careful that she won't pursue me with legal action after I'm done with the defenceless sub. After vocally confirming that she's fine with I doing anything to her, I'd blindfold her and start teasing her sensitive areas. Maybe tickling her intensively just for giggles (pun intended
). After the fun, I'd start painful torture such as impact play, nettle play, predicament scenarios and the possible involvement of insects.
When I get tied up naked then I want to be blindfolded with sound-blocking headphones so I have no idea who sees me and what they might plan on doing to me. The best way to be tied naked (e.g. in a public park) is tied dangling from a strong branch of a tree, about a metre high from the ground. Genitals exposed, legs spread, arms rendered useless, mouth forced open, etc. The best thing would be not knowing what to expect and who used me, either men or women.