AES multiple devices to server communication
Will the presence of this "secret" (presuming it is the same in every message coming from that device, and in the same location within the message) cause repeating patterns in the ciphertext making some form of attack possible?


As long as the AES mode uses an IV and the IV is different for each message, there will be no patterns in the ciphertext.

However, there may be other attacks.


  As this is sent unencrypted, I think this is somewhat insufficient to identify the device so I placed a device specific "secret" in the bit of the message that is encrypted which, if absent or incorrect will lead to the server discarding the message.


A better way to do that would be to use authenticated encryption. Either AES in GCM mode or AES CTR + HMAC would work, for example. That will also prevent changes to messages, which may currently allow an attacker to change the device status in a message.

Even then, replay attacks may be possible. To prevent those you could verify the IV is never reused, for example by using a counter as the IV (in CTR or GCM modes) and storing the last value on both server and device, or by having the server send a random session number the device needs to include in its message.