Harlikwin's right.
Just like you should treat a gun like it's always loaded, treat a NVD like it's always turned on.
You will spot many threads here about the damage people have accidently caused by leaving their NV turned on, cap off. :(
You can also damage it cap on, but it won't be as bad for the same amount of exposure and there's a better chance you can fix it.
Though from a technical perspective, the photocathode of a Gen3 tube is just a chunk of semiconductor material and it's not affected by light when it's powered off... If you do want to play with it, then make sure the batteries are OUT before you do go opening it up or looking at it without caps in daylight.
Accidently switching it on in daylight would be a bad thing :(
Also, never point it towards the sun while the objective lens is in place. Ever... That's always a bad thing. Think about "ants" and "magnifying glasses" to understand why.
David