I am no lawyer (but I play one on TeeVee…or at least wish I did), but I suspect that things get a bit more "gray" if you create that "something" on your own time (i.e. at home) and most likely on your own resources. It is definitely "gray" if you do it on your own time but using the company's resources (especially if you don't say own TEDDs or MathCad yourself). I believe if you are not using any paid company time or company resources, then it still will be your IP, but then as I said, I ain't no lawyer.
Now, in the practical world, it gets a bit more complex. For a lot of that stuff, it is going to be rather hard to prove that company A has company B's IP. After all, it is not like you can make general notes, typical details, many structural spreadsheets, etc "unique" enough that you would be able to prove that they "stole" your IP. fsubb is still M/S, etc, no matter how you spin it. It would generally take a REAL unique piece of IP for a company to go after a previous employee or some new company that is employing that employee now…at least in my opinion.
And beyond that, for most of this stuff, you generally will have "mutual assured destruction" coming into play. After all, there is a darn good chance that company B has some of company A's IP while company A has some of company B's IP.
So to me, the reason why you don't see companies getting their panties in a bunch over this kind of stuff is that they realize for the vast majority of such "somethings", they would have a tough time proving it was their and they also know that they likely have someone else's "somethings" floating around their company. So, they realize that engaging lawyers and suing people would likely do nothing other than make the lawyers rich.
Now, I can imagine the for some isolated cases with some really unique "somethings", then it might be worth it. One company that I worked for had created a proprietary program (i.e. true, "hardcore" programming to create their own DOS program). I forget what the program was for as it was so long ago. But, I could imagine that they might pursue someone for using that program outside of the company.