On occasions we have found our front door or back door to be open when we know they have been closed.To-day I saw the front door open by itself. What do you think is happening.

Check your locks and maybe get a dead bolt installed. If it opens after that with a dead bolt in "locked" position then let us know.

Could be a spirit... could be air flow moving your door. That's not really something we can answer over the internet with the details provided. Were the doors latched or locked?

Set up a video camera with a motion trigger (most web cams can be set like this) and see if you can discover what is going on.