Yes, there are multiple frequencies in the 2.4 GHz band. Let's assume you have an installation with 20 sensors and each is on a different frequency. You will need 20 fixed-frequency Wixel's to detect them, as there is no scanning ability built in. Now, since you are dealing with smart robbers, they bring a blocker that transmits on each frequency. Therefore, you need an acknowledgement sent by the main receiver to the triggered sensor to say alarm received. You will need 20 transmitters to do that (or a system to scan the transmitter). Of course, the robbers don't worry about complying with the FCC, so they broadcast continuously and block the acknowledgement too.
How is that operationally different than having all transmitters and receivers on the same frequency using different addresses. For security, you use a typical transmitter/receiver combination that scans and binds to each other and frequency hopping (See:
Hedy Lamarr and composer
George Antheil, "Secret Communications System" ,
https://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum ).
Allowing each station to have its own coded identifier (maybe even 64-bit) will be more effective at preventing a would be robber from sending a fake "all is OK disregard alarm" than simply using 250 distinct frequencies. Of course, there will need to be a handshake between central receiver/transmiter and each sensor receiver/transmitter combination. But, you will not avoid that by using different frequencies either.
Perhaps the most productive next step would be for you to sketch out your proposal with a little more detail than given in your first post, and let us contribute to improving that design.
Edit: I would also consider a "blocking detector" so the alarm would sound if an expected ack signal is not received. My simple alarm system has that ability. The battery-operated senders operate for >3 years without needing to replace the batteries.
John