I see this too, in cold weather (my camera is outdoors). This seems to be caused by a heater in the camera that cycles on and off in cold weather, causing the camera to switch between day and night mode, and detect motion.
If I use a video motion trigger, I get this in the camera's log:
2014-03-25 02:12:51 SYSTEM SET HEATER HOLD
2014-03-25 02:15:29 SYSTEM SET IR LIGHT ON
2014-03-25 02:15:29 IP CAMERA Received MOTION Trigger
2014-03-25 02:15:33 MOTION STOPPED
2014-03-25 02:15:34 SYSTEM SET HEATER OFF
2014-03-25 02:16:47 SYSTEM SET IR LIGHT OFF
2014-03-25 02:16:47 SYSTEM SET IR LIGHT ON
2014-03-25 02:16:47 SYSTEM SET IR LIGHT OFF
2014-03-25 02:16:47 IP CAMERA Received MOTION Trigger
2014-03-25 02:16:50 MOTION STOPPED
2014-03-25 02:18:54 SYSTEM SET HEATER HOLD
2014-03-25 02:21:35 SYSTEM SET HEATER OFF
2014-03-25 02:24:30 SYSTEM SET HEATER HOLD
2014-03-25 02:27:03 SYSTEM SET IR LIGHT ON
2014-03-25 02:27:03 IP CAMERA Received MOTION Trigger
2014-03-25 02:27:06 MOTION STOPPED
2014-03-25 02:27:11 SYSTEM SET HEATER OFF
2014-03-25 02:28:26 SYSTEM SET IR LIGHT OFF
2014-03-25 02:28:26 SYSTEM SET IR LIGHT ON
2014-03-25 02:28:26 SYSTEM SET IR LIGHT OFF
2014-03-25 02:28:26 IP CAMERA Received MOTION Trigger
2014-03-25 02:28:29 MOTION STOPPED
2014-03-25 02:30:42 SYSTEM SET HEATER HOLD
2014-03-25 02:33:29 SYSTEM SET HEATER OFF
2014-03-25 02:36:16 SYSTEM SET HEATER HOLD
2014-03-25 02:37:47 SYSTEM SET IR LIGHT ON
2014-03-25 02:37:47 IP CAMERA Received MOTION Trigger
2014-03-25 02:37:50 MOTION STOPPED
2014-03-25 02:38:28 SYSTEM SET HEATER OFF
2014-03-25 02:42:53 SYSTEM SET IR LIGHT OFF
2014-03-25 02:42:53 SYSTEM SET IR LIGHT ON
2014-03-25 02:42:53 SYSTEM SET IR LIGHT OFF
2014-03-25 02:42:53 IP CAMERA Received MOTION Trigger
2014-03-25 02:42:56 MOTION STOPPED
If I use the PIR sensor, it does the same thing, the heater triggers the infrared motion detection:
2014-03-26 21:38:12 SYSTEM SET HEATER HOLD
2014-03-26 21:38:17 IP CAMERA Received PASSIVE-INFRARED-SENSOR Trigger
2014-03-26 21:40:46 SYSTEM SET HEATER OFF
2014-03-26 21:40:51 IP CAMERA Received PASSIVE-INFRARED-SENSOR Trigger
2014-03-26 21:43:50 SYSTEM SET HEATER HOLD
2014-03-26 21:43:55 IP CAMERA Received PASSIVE-INFRARED-SENSOR Trigger
2014-03-26 21:46:24 SYSTEM SET HEATER OFF
2014-03-26 21:50:01 SYSTEM SET HEATER HOLD
2014-03-26 21:50:06 IP CAMERA Received PASSIVE-INFRARED-SENSOR Trigger
2014-03-26 21:52:24 SYSTEM SET HEATER OFF
2014-03-26 21:52:29 IP CAMERA Received PASSIVE-INFRARED-SENSOR Trigger
2014-03-26 21:56:06 SYSTEM SET HEATER HOLD
2014-03-26 21:56:10 IP CAMERA Received PASSIVE-INFRARED-SENSOR Trigger
2014-03-26 21:58:45 SYSTEM SET HEATER OFF
"Heater Hold" seems to mean heater on. I only get this day/night switching when it's below freezing at night.
Due to this problem, I have the camera storing these motion events to SD only during the night, not sending them by email.
These cameras sort of do what I want, but it's a pain to get them set to do outdoor motion detection very well, and to deal with all the false alarms due to sun, wind, and shadows. Because of this, I've installed large memory cards in them, recording all day long, for review in case the motion detection misses things (even when it works, I find it useful to review more video in the minutes around the motion event).
My other solution for night is to disable the IR light entirely. I found with it on, the motion detection would go crazy in snow storms. As it doesn't illuminate very far anyway, but is great for lighting up snowflakes and insects right in front of the camera, I decided to simply leave it off on the one camera I'm keeping enabled for motion at night. I have a separate yard light with motion detection in that area, and that illuminates the scene much better when there's motion than the camera's IR light does anyway.