Also, could such an app detect a specific frequency (using either the cell tower antenna or wi-fi sniffer)? I would like to use a radio transmittor to activate an app to perform a function even while the phone is in standby mode. Thank you in advance for your assistance.
The cell radio in a mobile phone is designed only to receive and transmit at certain frequencies. It can't pick up an arbitrary transmission such as a radio car key, regardless of whether it's in standby or fully on. It's very unlikely that you could even do this by replacing the device's radio firmware, let alone with a normal app. At the same time, even if you can modify the transmitter, running it on a licensed mobile phone frequency would likely constitute a criminal offence.
The same goes for the Wi-Fi and Bluetooth radios too.