Sometimes it Beeps.... Sometimes it doesn't

[baseball]

Most of the time when I lock my 987 with the remote key, the lights flash twice and that's it. Everything's locked, life is good.

But every now than then, when I lock it with the remote, in addition to the lights flashing, the horn will also beep once.

Does anyone know why the horn would beep? Personally, I prefer it to beep, but don't know why it beeps intermittently.