How do I find my lost car keys?? I can't find them anywhere!!?

It's a 2010 hyundai elantra, and about a week or two ago I noticed my main set of keys went missing (I have a second set of keys too), at the time didnt think much of it because I didnt really need them then. But I checked the place I thought I left them last, they're not there. I checked all around my house and all my pockets I can think of. Any suggestions on how I can recover them? I have an iPhone 5, so I can download apps for it if needed. I don't have any Bluetooth tags or anything attached to the keychain to help locate them, but I'm pretty sure it has that "chip" that allows the car to recognize the key, though