redistribution in a non-compatible license is also a GPL violation. Source present or not. You can't release a forked GPL product under e.g. the BSD license
(I'd say it's that way from any reasonable perspective, too. There's nothing wrong with wanting to do your tinkering in private.)
And even for software that's being redistributed there's never been any requirement in the GPL that you have to make your source control public. The GPL's conditions can just as easily be met by distributing your software along with a tarball of the version of the source that was used to build it.
Section 6 from the GPL (distribution of non source-forms)
b) Convey the object code in, or embodied in, a physical product
(including a physical distribution medium), accompanied by a
written offer, valid for at least three years and valid for as
long as you offer spare parts or customer support for that product
model, to give anyone who possesses the object code either (1) a
copy of the Corresponding Source for all the software in the
product that is covered by this License, on a durable physical
medium customarily used for software interchange, for a price no
more than your reasonable cost of physically performing this
conveying of source, or (2) access to copy the
Corresponding Source from a network server at no charge.