A small incident at my neighbour's house got me interested in this. He bought a Faraday pouch after reading about keyless car theft and now keeps his fob inside whenever he's home. We tested the pouch with his spare key and noticed that the car didn't respond while the fob was inside. Once removed, everything worked normally again. There are a few other things worth knowing about keyless entry and car security, especially for UK drivers. For some background on vehicle access equipment, this car unlocker page is worth a look, as it gives some context about related devices. I'd still choose a properly shielded pouch and replace it if the lining gets damaged. A simple habit like that seems quite reasonable.