Everywhere I've worked has had an IP policy that was agreed/signed at the same time as all of the new-hire paperwork. The ones I've signed essentially said "if you develop it on your own time and equipment and it's not based/dependent on our code, it's yours; if you develop it on our time or equipment or codebase, it's ours". The others I did not sign. Anecdotal experience, probably not reflective of everyone's hiring practices, et cetera.