Are they XT2 cameras, and if so, are you making use of live view? Live view can be set to record, when using an XT2, and I suspect it may be the default setting. Browse through your settings, and see if you can find it. If not, others will be able to point you at it.
If they are original XT cameras, I have little idea. I’ve never yet had a recording from a camera with motion disabled, or if the system is disarmed. The only way I could think of this being a possibility is if more than one person has access to your system, and is arming/disarming, without you knowing.