Looks like a Smoke Detector!
This very compact round patch antenna provides 8 dBi gain with very broad coverage. It is suitable for both indoor and outdoor applications in the 2.4 GHz ISM band, including IEEE 802.11b/g, Bluetooth and public wireless hotspot applications. This antenna is lightweight and features an aesthetic UV-stable white plastic radome. It can be installed for horizontal or vertical polarization. Can be wall or ceiling mounted.

The HG2408P can be mounted flat against the wall. Its unique design allows the cable to exit out of the side or from the back of the unit which is ideal for mounting in locations that require the cable to be hidden.