Yes, that's in the big list of items to consider at some point.
All leaders present in the exploring fleet will get a fixed amount of XP, depending on if they find something on ruins or not.
Spy leaders gain XP continuously during the set up phase. Intel-gathering leaders receive even more XP when they are in effective spy duty. All spy leaders also gain bonus XP when their action is taken. This also applies to intel-gathering leaders, applied when they effectively start their intel-gathering activities.
I's based on what gets done in their colonies.
We have a loading window hint about how leaders gain experience. I think you may find this helpful. It goes like this:
"Leaders gain experience in several ways. Every turn from instructor leaders, buildings and others. Colony leaders from completed projects in their colonies. Ship leaders from fighting battles, exploring systems and searching ruins."