There’s actually a decent amount of research into exactly this sort of thing, called, appropriately, “energy harvesting”. Depending on the application, it can be fairly effective. I know that vibration energy harvesting has been successfully in machinery monitoring applications, for example. I haven’t looked much into stray EM harvesting, but I’m sure it is possible…most likely to supplement a primary energy source rather than as the main source itself. But, yeah, for sure it’s an interesting topic with a fairly good amount of study into it…I think the University of Michigan had a few good research programs related to harvesting for ultra low power applications. You night check out some of what they’ve done