If the players know what the hand signs mean, they would know what they mean.
>The party sees a figure in dark cloths, barely visible amongst the shadows of the alley opposite you next to a barrel and strewn refuse. In the low light, you can see him making hand gestures which you (PC A) recognize to be a warning: "guards approaching left"
If they don't, they could clearly see SOME sort of hand signs. It's up to them to infer or to react from there - whether they see it as some kind of warning, whether they try and whisper to that person, whether they fumble it and approach them and are noticed by the guards should play out as it happened.
>The party sees a figure in dark cloths, barely visible amongst the shadows of the alley opposite you next to a barrel and strewn refuse. In the low light, you can see him frantically making a string of hand gestures that none of you seem to recognize.
pic unrelated