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.
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.