Yeah, it is your cell number. If Hero have put your cell number in their phone’s contacts, and given Facebook full access to their contact (which includes you), then Facebook will put you in their list of “people you may know”, because you also have given your cell number to Facebook and you have not not used the privacy settings that hide it under all circumstances.
And no it is not location, just being next to someone on the bus doesn’t mean you know that someone, but giving your number to someone, or calling them, that means you likely know them, so Facebook uses phone numbers, not your gps location on your phone.