Aside from the smart aleck answers that sprang to mind, here is a serious one...
Since your TV can be turned on and off by a remote control, it is possible that something that occurred in the room activated the TV. For example, the jingling of keys can operate some remotes. A scrape of something on the floor might have hit the right frequency. Etc. Now that most remotes are infrared, we don't have that problem so much, but with older sets, it was a problem.
Another possibility is that the batteries in the remote are wearing down, and as they do so, they can trigger "one last gasp" and turn on the TV as the batteries produce uneven power due to the weakening.
A third possibility is that the Power button on the remote or on the TV is stuck in a partially-depressed state, and it will randomly operate the TV as it "shorts out" in one direction or the other.
Finally, if your TV has a TIMER setting on it, then it may have been set at some point to turn on at a certain time, and it has been doing that for a while, but you only found out because you were in the room at the right time.
P.S. Is it a Samsung Smart TV? There have been some issues with them turning on/off based on turning on other devices that are either intentionally or unintentionally connected to the TV. That could be a simple unplug/wait a while/plug back in fix, or it might be a defective remote or other device. If it's a Samsung, you might want to do a more detailed search to get more info.
Good luck ... from ... the Ghost of TV Past... ;-)