In a situation where I need to quickly address someone I don't know, how can I do so without using gendered language? For example, as a retail worker, if a customer was to leave their credit card behind I'd naturally say "Excuse me! Sir! You left your card behind." Or if I were a steward on a train I'd naturally say "Ma'am, Please move your bag from the aisle."
If I need to get someone's attention, don't know that person's name, want to be polite, and want to avoid "Sir" or "Ma'am" because maybe the person isn't either, what do I say?
This question was prompted by a Facebook post from a non-binary friend who got agitated about being addressed as ma'am while on a train. This got me thinking about what possible non-binary/gender-neutral alternatives there are out there. I'm specifically talking about respectfully referring to people whose names you don't know.
My friend and I are both based in the UK but answers for other cultures would also be appreciated.