Yeah I do this. Most of what I build, nobody knows about it nor is it release into the wild. Latest example: at my current workplace we have some crappy custom compilers for our platform.... I've rebuilt them in a way that I like (and using C# instead of Java), that enables me to use it in a small visual studio code plugin that enables me with real time error messages as I code against our platform, so basically cons…
Why would they have to pay you a cool sum of money for it? It sounds pretty clearly like you did it for work, so unless you have a very unique employment contract, they already own it.
If I use my own time to make something, work isn’t getting it without paying for it. I don’t care what the contract says about personal projects (although I‘ve also never signed one of those “we own everything you do outside of work” clauses because I find them exploitative and abusive)