If they ask you for your salary you have 3 options :
1. Withhold the information - Which in turn may cause some of them to not want to deal with you (this happened to me!). Some HR people are some real fickle shits. They know what they can give you but you still have to play with their rules. Remember that a lot of HR positions/headhunters are low education with lots of panhandlers in between.
2. Tell it as it is - Which in turn may cause them to short-change you as you mentioned. However, if this happens prior to the interview, just go along with it and have the client meet you and show them what you're all about. If they're still interested post-interview.... then you can negotiate! "I'm very excited to work for X, however, after some thought, I will be more comfortable if I'm making Y". Then state your reasons. If you already have a job, HR people HATE you. At this point, you're not dealing with HR, you're dealing with them indirectly through your future boss. They got a budget and will be more flexible.
This is how I got my current job. We offer X. I'm looking for Y with 15K difference. But I'll meet the manager, it actually was a job in Laval which in turn makes it a 10 min commute instead of possibly 1+ hour. Met my boss, she loved me. Hired at the salary I wanted. Now I make more. I could make more elsewhere but let's be honest, 10 min commutes are the bomb.
3. Lie - Which in turn may cause big issues if they find out you're lying or are asking for too much money. If you exceed their budget with that 1 quote, you're OUT.
I personally prefer to say it as it is.
"This is how much I'm doing, this is what I'm looking for because I bring X and Y to the table."
I'm currently happy at a job I'm at, I'm not leaving for less than a 20%+ increase plus better benefits.
Sometimes you have to play ball but at the same time ensure you keep your balls which you negotiate.
If they're talking to you and reaching out to you, you're already a candidate.
They now want the best one they can get for the budget they have to work with.