The PTCRB website shows that the Xperia M has firmware build 15.4.A.0.21 certified, which follows the last update with build 15.1.C.2.8 (another update 15.1.C.2.9 was certified yesterday but we do not expect it to be released).
The move from 15.1.X.X.X to 15.4.X.X.XX suggests that this is a big update. We already know that Vodafone Aus was scheduling testing of Android 4.3 firmware for the Xperia M, so there is a good chance that this could be it. Xperia M has been languishing with Android 4.1.2, whilst the Xperia M dual is currently running Android 4.2.2.
The Xperia M dual PTCRB certification has also seen a similar jump in build number from 15.2.A.2.5 currently to build 15.5.A.0.16. Once again a very big template jump from 15.2.X.X.X to 15.5.X.X.XX. Another update was certified yesterday with build 15.2.A.2.14, although we don’t expect this to be released. Hopefully, this points to good news for Xperia M owners and perhaps an Android 4.3 update soon.
Thanks difwwh, @KaneshiroAngga and rickir5!