There are a large number Unicode characters that may qualify, but
unfortunately "invisible sign" is not a defined technical term and it's
difficult to guess what you would or would not include under that term.

Instead, perhaps you should look over the Unicode concept of
"categories" and decide which ones you are interested in.  A complete
list is available at:

   http://www.dpawson.co.uk/xsl/rev2/UnicodeCategories.html

Initially, it looks like you'll be interested in all the Z categories,
and perhaps some or all of the C categories as well.  See also
Character.getType, which lets you determine which of those categories a
specific Unicode code point belongs to.