I accidentally saw a spreadsheet showing salaries, and I'm earning much less than coworkers with similar responsibilities. Do I admit how I found out, or focus only on market rates?

Having seen this information, you have the following questions to answer yourself:
1. Am I going to do anything about this or just get used to the... fact that I'm paid less than others?
2. Do I draw up a battle plan to assess myself against others at my level and go for a raise or do I plan my exit?
3. Do I leverage the fact that I have seen this information or keep that fact to myself? If I choose to leverage it, how and with whom?
4. Does the degree of insincerity and contempt for workers that this information reveals tell me I made a mistake coming to work here or would I do the same thing if I were in their shoes?

Once all three questions are answered sincerely, move forward immediately.

No company is going to pay anybody one dime more than they have to. If they can get a steal of a deal on your smarts and hard work, they will. Business has no morality. Never has. more

A recruiter asked me for my current salary before discussing the role. Is there any benefit to answering honestly?

JD Walker is absolutely correct. I myself have gone even stronger at times. Recruiters are loathsome in their gall. I would have no problem saying... something like this:
"Let's get something straight. We are talking as equals. I have salary requirements and I am eminently qualified. Do not ask me what I am paid now, that is NONE of your business. Now can we get on with this interview or shall I just hang up?"
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