I did this at the convention this year:
1. Downloaded a 3rd party app and named my hotspot jehovahs-witness.com during lunch time
2. left it open
3. walked around mingling and doing my 'attendant' duties
4. found out that 97 devices out of a crowd of 5000+ had 'connected'
I'm not sure how many of those were deliberate or which ones were set to automatically connect to available networks.
The method makes sense though. Someone can receive an 'unwitness' even months after while going through their Wifi connection list.