I noticed that if you make paint a square (not filled) the sprites inside it can sense it.

Yeah, it seems to be a bug, similar to overlayed shapes on a ghosted sprite looking derpy (try it out!)

TPAL125 wrote:

I noticed that if you make paint a square (not filled) the sprites inside it can sense it.

Hmm, it mightbe vector issue. Even if a closed object has a transparent fill, that transparency is still an object fill .

I did a quick test. http://beta.scratch.mit.edu/projects/10041695/ It had some interesting results
The sprite shows sensing inside a closed box with transparent fill. Same is said for any closed object, such as a circle.
The sprite shows no sensing in a an open box where the fill has been removed.
The sprite shows no sensing inside a closed box made using the bitmap editor.
The sprite shows no sensing at all for a transparent open vector box!