Bluetooth or Wi-Fi or other protocol signal detector
Yes, Bluetooth can of course be used for ANY generic data communication; what you put in the transmitted payload depends on your application -- in this case, just a simple greeting.

Regarding your comment about "air signals", I don't understand exactly what you are saying, but assuming you are talking about interference effects: While there may be some interference of the RADIATION due to coexistence of two similar-frequency networks, and thus increased latency, you can rest assured that there will NOT be any degradation in terms of what DATA your receiver actually squeezes out, because the various transmission protocols are robustly designed in terms of structural uniqueness, collision avoidance, checksums, channel-hopping, etc..

Your main hurdle, if anything, will be running the receiver with battery power (again, I'm assuming that is true). For example, if you want to keep your receiver listening ALL the time, for any message from the transmitter, then you will have to keep the receiver's Bluetooth/RF module powered all the time.

So, if you want to cut down on current draw, you would do well to rely on a sleep cycle, e.g., you can have the receiver wake up every X seconds, send out a greeting to the transmitter, see if transmitter responds immediately, otherwise, go to sleep.

Also, you might first want to do a power consumption estimate/comparison to see whether the power savings, obtained from your plan of waking up the microcontroller on cue, are even worth it, compared to how much power is drawn by the RF receiver. Because if the latter draws more power than the microcontroller, then you might be better off simply keeping your microcontroller on all the time, possibly even refined further by judicious use of sleep -- e.g., if you working with a MSP430 or any of the highly efficient microcontroller series, you can get &lt; 1 microamp draw in its sleep.

Alternatively, if the transmitter and receiver are in line of sight, you can instead just use infrared (IR) transmission -- easy and inexpensive.