[ColonelPhantom]

And then I will download a trial of the resulting binary, and send a GPL compliance letter to the company. Unless they took care to use dynamic linking in the LGPL case, they are legally obliged to send me the source code under the license, so I can release it all as FOSS under that license.

[charcircuit]

Yes, they are legally obliged, but they can just tell you to pound sand and you can't do anything because you aren't the copyright holder.

[ColonelPhantom]

Then I contact the copyright holder (or one of the many, in case of e.g. the Linux kernel). They probably care, or else they would not use the GPL. I also believe there are organizations that can help such as the Software Freedom Conservancy.