If youre doing extra work in your own time you still have to declare the work (moonlighting folks... bad mmmkay) to your employer due to the working hours statute. If it is deemed they have a vested interest in the product youre producing then they can terminate your contract of employment, or if it comes out that intellectual property of theirs was used then they can prosecute you for damages.
Ive already signed a waiver for EU working hours regs & anyway contributing to open source stuff as a hobby is not work any more than reading a maths book or watching a documentary.
Im inclined to agree with what beaker has said ref IP rights as i know full well most of the anti poaching agreements & enforced unemployment periods (unless theyre still paying) cant be enforced within the EU as theyd be denying you the right to work. The only issues I can see are, as youve pointed out, where I was say developing something similar in house and then contributed to a similar project - obviously there would be a conflict and they could reasonably argue that my experience/knowledge gained with them was being used etc... but tbh.. since Im not a developer at the moment this isnt an issue.
As for working in the open source arena, there are way too many bad practices to deal with that go on out there that to get involved in my opinion would actually set me back in my development to be honest.
Thats a bit of a big brush to tar the whole open source movement with tbh... Ive done my research and know what Im interested in getting involved with btw..- Im not just going to pick some random projects that fit my areas of interest.