Very interesting.
On the Mountain Lion front, I am still seeing these entries in the install.log on an irregular basis:
Jun 1 00:59:14 Als-iMac-i7.local Software Update[56778]: SoftwareUpdate: Checking for critical updates only.
Jun 1 06:00:33 Als-iMac-i7.local Software Update[57099]: SoftwareUpdate: Checking for critical updates only.
Jun 1 09:38:37 Als-iMac-i7.local Software Update[57410]: SoftwareUpdate: Checking for critical updates only.
Jun 1 12:59:15 Als-iMac-i7.local Software Update[61708]: SoftwareUpdate: Checking for critical updates only.
Jun 1 17:41:23 Als-iMac-i7.local Software Update[6080]: SoftwareUpdate: Checking for critical updates only.
Jun 1 18:42:01 Als-iMac-i7.local Software Update[6518]: SoftwareUpdate: Checking for critical updates only.
Jun 1 21:41:23 Als-iMac-i7.local Software Update[7935]: SoftwareUpdate: Checking for critical updates only.
Jun 1 22:42:01 Als-iMac-i7.local Software Update[8359]: SoftwareUpdate: Checking for critical updates only.
Jun 2 01:41:23 Als-iMac-i7.local Software Update[9747]: SoftwareUpdate: Checking for critical updates only.
Jun 2 05:29:56 Als-iMac-i7.local Software Update[10022]: SoftwareUpdate: Checking for critical updates only.
Jun 2 05:41:22 Als-iMac-i7.local Software Update[10203]: SoftwareUpdate: Checking for critical updates only.
Jun 2 07:40:11 Als-iMac-i7.local Software Update[10538]: SoftwareUpdate: Checking for critical updates only.
Jun 2 09:41:23 Als-iMac-i7.local Software Update[12965]: SoftwareUpdate: Checking for critical updates only.
Jun 2 11:33:57 Als-iMac-i7.local Software Update[15067]: SoftwareUpdate: Checking for critical updates only.
Jun 2 13:41:23 Als-iMac-i7.local Software Update[15955]: SoftwareUpdate: Checking for critical updates only.
Jun 2 14:42:01 Als-iMac-i7.local Software Update[18335]: SoftwareUpdate: Checking for critical updates only.
Jun 2 17:41:22 Als-iMac-i7.local Software Update[19511]: SoftwareUpdate: Checking for critical updates only.
Jun 2 18:42:01 Als-iMac-i7.local Software Update[19906]: SoftwareUpdate: Checking for critical updates only.
Jun 2 21:56:37 Als-iMac-i7.local Software Update[20830]: SoftwareUpdate: Checking for critical updates only.
Jun 2 22:42:01 Als-iMac-i7.local Software Update[21204]: SoftwareUpdate: Checking for critical updates only.
Jun 3 01:41:22 Als-iMac-i7.local Software Update[22347]: SoftwareUpdate: Checking for critical updates only.
Jun 3 02:42:01 Als-iMac-i7.local Software Update[22710]: SoftwareUpdate: Checking for critical updates only.
Jun 3 07:40:12 Als-iMac-i7.local Software Update[22781]: SoftwareUpdate: Checking for critical updates only.
Jun 3 09:41:23 Als-iMac-i7.local Software Update[24572]: SoftwareUpdate: Checking for critical updates only.
Jun 3 10:42:02 Als-iMac-i7.local Software Update[27020]: SoftwareUpdate: Checking for critical updates only.
Jun 3 13:41:23 Als-iMac-i7.local Software Update[28377]: SoftwareUpdate: Checking for critical updates only.
Jun 3 14:42:37 Als-iMac-i7.local Software Update[28612]: SoftwareUpdate: Checking for critical updates only.
Jun 3 16:50:28 Als-iMac-i7.local Software Update[30544]: SoftwareUpdate: Checking for critical updates only.
Jun 3 17:41:22 Als-iMac-i7.local Software Update[30802]: SoftwareUpdate: Checking for critical updates only.
Jun 3 18:42:01 Als-iMac-i7.local Software Update[31173]: SoftwareUpdate: Checking for critical updates only.
Jun 3 20:59:58 Als-iMac-i7.local Software Update[32036]: SoftwareUpdate: Checking for critical updates only.
Jun 3 21:41:22 Als-iMac-i7.local Software Update[32304]: SoftwareUpdate: Checking for critical updates only.
Jun 4 23:50:43 Als-iMac-i7.local Software Update[11412]: SoftwareUpdate: Checking for critical updates only.
Jun 5 04:45:21 Als-iMac-i7.local Software Update[13043]: SoftwareUpdate: Checking for critical updates only.
Jun 5 07:50:43 Als-iMac-i7.local Software Update[14668]: SoftwareUpdate: Checking for critical updates only.
Jun 9 18:51:35 Als-iMac-i7.local Software Update[15422]: SoftwareUpdate: Checking for critical updates only.
Jun 10 18:51:36 Als-iMac-i7.local Software Update[28650]: SoftwareUpdate: Checking for critical updates only.
Jun 10 22:51:35 Als-iMac-i7.local Software Update[30190]: SoftwareUpdate: Checking for critical updates only.
Jun 11 02:51:35 Als-iMac-i7.local Software Update[31537]: SoftwareUpdate: Checking for critical updates only.
Jun 11 06:51:34 Als-iMac-i7.local Software Update[32846]: SoftwareUpdate: Checking for critical updates only.
Jun 12 10:51:36 Als-iMac-i7.local Software Update[65367]: SoftwareUpdate: Checking for critical updates only.
Jun 12 14:51:35 Als-iMac-i7.local Software Update[66883]: SoftwareUpdate: Checking for critical updates only.
Jun 12 18:51:35 Als-iMac-i7.local Software Update[71662]: SoftwareUpdate: Checking for critical updates only.
Jun 12 22:51:35 Als-iMac-i7.local Software Update[72728]: SoftwareUpdate: Checking for critical updates only.
Jun 13 02:51:35 Als-iMac-i7.local Software Update[74188]: SoftwareUpdate: Checking for critical updates only.
Jun 13 06:51:34 Als-iMac-i7.local Software Update[75326]: SoftwareUpdate: Checking for critical updates only.
Jun 13 10:51:34 Als-iMac-i7.local Software Update[90556]: SoftwareUpdate: Checking for critical updates only.
Jun 13 14:51:35 Als-iMac-i7.local Software Update[95046]: SoftwareUpdate: Checking for critical updates only.
Jun 13 18:51:35 Als-iMac-i7.local Software Update[2321]: SoftwareUpdate: Checking for critical updates only.
Jun 13 22:51:36 Als-iMac-i7.local Software Update[3337]: SoftwareUpdate: Checking for critical updates only.
Jun 14 02:51:35 Als-iMac-i7.local Software Update[4612]: SoftwareUpdate: Checking for critical updates only.
Jun 14 07:40:18 Als-iMac-i7.local Software Update[5933]: SoftwareUpdate: Checking for critical updates only.
Jun 14 11:38:00 Als-iMac-i7.local Software Update[10521]: SoftwareUpdate: Checking for critical updates only.
Jun 15 11:45:25 Als-iMac-i7.local Software Update[24331]: SoftwareUpdate: Checking for critical updates only.
Jun 15 16:56:32 Als-iMac-i7.local Software Update[26854]: SoftwareUpdate: Checking for critical updates only.
As you can see there varying number of checks and large gaps with no checks at all (June 6-8 and 16-now). I'm sure some of it is due to computer sleep (set at one hour), but that doesn't come close to explaining all of it.
I just double-checked and the latest XProtect update is still May 10, so you haven't missed anything there. I've gotten a check recorded every day around 09:24 local in every system.log for the last nine days.