Access points generate a unique MAC address for each different SSID and each radio, known as a BSSID. This distinction is necessary so client devices are able to communicate to the proper SSID at Layer 2. It is possible to identify 802.11 traffic from a specific access point by finding these BSSIDs.
Note: Different models of access points increment their BSSIDs differently, but all following a similar pattern; the 3rd octet will increment for 5GHz vs. 2.4GHz, and either the first or last octet will increment with each SSID.
The image below is a screenshot of an MR16 Access Point with 4 SSIDs enabled, it has a physical address of 00:18:0a:31:0a:5e.
The image below shows the various MAC addresses that are seen for each SSID. By viewing the broadcast traffic using a wireless sniffer, such as third-party tool like inSSIDer, we can see this information. In this example we see two instances of each SSID, which correspond to the 2.4GHz and 5GHz ranges that the MR16 operates in:
On the MR16, only the first and third octet of the MAC address change. The first octet changes as a way to differentiate SSIDs from each other. The third octet displays as 0a for a 2.4GHz signal, and 1a for a 5GHz signal. The rest of the MAC address remains the same, which makes it feasible to associate a BSSID with a physical access point.