When you get paid to do research, but you're not a graduate student/PI, is it mandatory to put in extra time? When I was an undergraduate student volunteering in laboratories, I recall times where I would come in on weekends to work on plating cells, running experiments, etc. This was something I did on my own accord because I was getting professional development. I notice the same phenomena exists among graduate students as their whole existence in graduate school revolves around the acquisition of knowledge, and primarily, the completion of research.

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However, what about a tech? Is a hired research technician obligated to stay more than the 40 hours a week their paycheck warrants? I am a newly hired tech and I was surprised/understanding to see my peers drop everything at 5 PM to resume the next day. Obviously this is not taken literally, but once stuff wraps around at maybe 5:10, 5:15...they're out the door. No one is staying until 7 PM, etc. like in college to make sure this assay runs well, etc.
A: Ask a labor or employment attorney in your local area

B: No. The law in most western economies is structured so that you have some form of redress (union, contract, employment law, etc.) so that you cannot be forced to work longer than a specific or nominal work week which can range from 35 to 40 hours per week (in my experience) without additional pay. Anything more is slavery and/or illegal employment practices. Now that doesn't mean that a savvy employer can't find a fake reason to fire you if you don't work enough hours to satisfy them. In lots of places that have "at will" employment (many states in the US, for example), you can be fired for the color of your shoes, your smell, etc., except for protected classes like race, gender, religion, country of origin, etc. Still savvy employers can find ways around this, even if you have an employment contract. 

What's you're real question? Are you worried about expectations from an employer, are you under the gun to work harder, or something else? Typically, unless something is weird, you're allowed to voluntarily work more hours than the standard work week, but you also can't be forced to without receiving legally mandated overtime pay. Some unscrupulous employers can even find ways around this, like by doctoring your time records, but this usually highly illegal and worth reporting to the correct authorities. Some classes of employees in the US are exempt from overtime, but a technician wouldn't often fall into that class in my experience.