One time I was riding to work, a group came from behind me and one of them started shouting “left left left”. We were just coming up to an intersection and I still don’t know what he meant: did he want to pass me on the left, did he mean to tell his group to turn left, or did he want me to go left?

Get a decent bell - not one of those little ting-ting ones that you can’t hear - and use it. A bell is always clear: someone wants to pass on the left (right in the UK).