Yes, unplugging works and I've done that with the ones that are easy to unplug (I guess that rules out ghosts). I still have two that are not easy to unplug. Plus, unplugging means going through setup again when the owners return. I don't think it's a sleep timer; but, perhaps, some type of energy saver ...
Consider that unplugging it helps provide additional protection from surges etc.. I think I would put up with going through the setup again. I'll bet you get really good at it.
Had a TV that would go on---it seems that it was very responsive to power surges. Since today's TVs are usually powered all the time, even when off, why not just pull the plugs on all the electronics--safer that way. MLD
Try this: Take a small square of black electrical tape and use it to cover the remote sensor of each set. Test with a remote to make sure you've blocked it. This will eliminate the possibility that someone is coming and shooting a remote through the window just to create mischief. (Obviously any weird things happening with the owner's own remotes can be eliminated by putting them in a drawer.)
Also test the theory that any of these TV's come on if power is dropped and restored. Probably not but worth testing to see if they power on when plugged back in.
If it was just one TV doing it and no other cause is found I'd say some sort of power surge could be the reason but with different ones (are they different makes and models?) that would seem less likely.
Make sure you leave a few little secret telltales around so you can tell if someone has been in there. If something turns up that way and there's an internet connection on these premises it might be fun to set up a hidden cam of some sort, maybe a network camera for which you run motion sensing monitoring software back home.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.