For example, what happens if you build a system whose maintenance relies on continued use of that IP after you leave? Are they forced to abandon the system that you built for them as part of your job? Since you could (or should) have known about this in advance, your voluntary use of this IP could possibly- if not probably- be construed as some form of implicit offer and/or agreement. What if they then want to sell that system commercially? What if they *only* want to sell it commercially because it lets them use- and build upon- your IP on the same terms as a work for hire?
You sue them for not having a license!
That seems to be how things work in the big industry world..