There are actually bags designed for shutting out phones. AFAIK they are mainly aimed at concerts to prevent people recording etc. So you drop your phone into a small bag that is a Faraday cage. The bag is closed with a very strong magnet (similar to theft prevention tags in stores. You then carry your phone with you without being able to use it, but also without anyone else being able to temper with it.
That said, I'm surprised why anyone wouldn't trust the wardrobe at a ycombinator event.
High-value targets, interesting technologies to steal, probably some shady secrets about some VC parties that could destroy some VCs or companies, source-codes, client data and trade secrets, info about unreleased products, plenty of reasons.
Pulling out a stunt: "Look how many naive people gave us our phone, buy our security solutions to prevent physical tampering. We didn't even have a logo, we just drew on a paper and pen."
That said, I'm surprised why anyone wouldn't trust the wardrobe at a ycombinator event.