Magic! Or numbered envelopes. Either way.
Would you trust your phone at the venue with employees? Would you trust them to return your phone and not give your phone to someone else accidentally that could then have your personal information if they leave the venue?
There's all kinds of security problems with this in a way. This "solution" works because it stops the person from being able to use their phone (should go a step further and prevent them from being able to take calls/texts in the area. Which from what it sounds like it should do but I don't see any mention of RFID blockers that are similar to the bags that police use to keep cell-phones from being functional on a cell network as evidence is collected) WHILE keeping their phones on them so no information loss/security risks.
ever been to a coat check?
Coat check is slightly different given they tag those and are smaller amount of people. You can tag the phones, sure. But given how many people are going to be in a venue (1,000+) stickering their phones AND going through 1,000+ phones seems a scaling problem.