Do hidden networks send beacons? I have seen hidden networks in backtrack and Ubuntu using airodump-ng. The output is weird like the power is shown as -1. Why does this happen?
As you can see here there are no beacons for a hidden network.
Yes, hidden network still send a beacon frame, just without the SSID.
Your wireless hardware sends the estimated received power level to Aircrack-NG. It could be due to unsupported hardware or your hardware is not able to come up with a good power figure for the signal. Check out this forum for some older info, and look at their hardware compatibility list
The beacons are still sent, however the SSID field is set to NULL. You can learn the SSID when a client is in fact connecting. This can be done through being patient, or by spoofing a deauthentication frame coming from the client.
Instead of increasing the security posture, I think that the security is instead weakened by turning off the SSID broadcast. What is instead happening is that clients will start beaconing for the network with the appropriate SSID EVERYWHERE THEY GO. So your clients, wherever they may be, will be asking for the appropriate network everywhere, potentially revealing information about your clients that they don't want to share. E.g. if you've been on a "business trip" and visited a wireless network of the name "Shady Lady Ranch". Uou don't want your colleagues to know you visited there, however they can simply sniff the traffic when you arrive back at the office, and they will see you probing for the network, revealing that you've been previously connected.
All in all, the beacons are still being sent, however the job to specify the SSID is now burdened on the client instead of the AP.