Just to cover your bases, run it by your pediatrician. There are a type of seizure that looks like zoning out and are usually diagnosed in the early years of elementary school. A child just spaces out for a few a short bit and doesn't really have anything else notable to parents. A girl I know well had this issue and it just appeared as if she drifted off.
Some kids are also low energy in school and have focus problems related to the sensory system.
Most likely it is absolutely nothing but since it is worrying you, talk to your doctor about it. good luck
I agree with the others - we used to call this "going on Standby". Used to be (maybe still is true) video cameras left "on" and not touched or moved for several minutes would click to "standby mode" to conserve battery.
When she's forced to sit in one place and not allowed to amuse herself, she goes into standby. Her mind goes somewhere else that's more interesting and productive than the boring activity that's happening in the present.
My guess is, when she's allowed free time she rarely feels bored or says she's bored. She's probably very good at finding little things to do to amuse herself, and is quite content doing just that.
But when she's forced to sit in a chair for too long with nothing all that interesting going on, she checks out in search of something interesting to think about.
I will say, this will probably continue to be a problem for her in her school years because small children are expected to sit still which isn't in most of their natures.
But cheer up, there's a light at the end of this tunnel and she will probably mature into a very creative and productive adult, and in the meantime she can learn strategies like sitting close to a kid who's always on task and occasionally checking to see what they're doing.
Spacing out' as you called it is very common in children when they are bored I have known many gaze out of windows and not focus when they were not interested in the subjects being taught, I don't think it means there is any problem..often they are day dreaming ..If you are concerned speaking to her Doctor may make you feel better .How old is she ?
How old is she? 3- 4 ? I can't imagine that at this point she would have a serious condition. However, does she get enough sleep at night? Maybe she overly tired and needs to sleep earlier.
If you are really concerned I would definitely talk to her pediatrician, but I dont think you should jump to any drastic conclusions just yet. Talk to your pediatrician.