I doubt any firm has a group that solely pipelines senior candidates. It's part of the role in good teams (sometimes a significant part, more so in leaner times) but there isn't a consultancy group I've heard of who would subsidise a Talent Acquisition function to this extent.
You are "in the market" and letting people know. Now you have received two approaches from presumably relevant firms. Given your post, the first question you should be asking is why they have contacted you, and you should be asking it of them. I'd be interested to hear the answer.
More generally to get on the radar of good people who perform this function for companies, there are a few points to it. It's a new function in many organisations and often still evolving. I've been watching this forum to scan the gossip for years without commenting but the previous response was a bit wide of the mark and aimed at attracting generic recruiters.
To temp-profile "Executive" search consultant suggesting that you update your LinkedIn profile with terms laymen can understand.. sure, post your CV on Monster while you're at it.
1. Be good at your job. Whatever the role is, eventually quality is recognised if point two is satisfied.
2. Be liked by peers and clients. They're the ones whose recommendations carry the most weight.
If you can do those two things a good internal recruitment function will eventually find you.
If you want to speed the process up;
Write white papers and blogs that contain some insight (we read those as well, probably more so if you are at a bigger firm)
Attend relevant conferences (we're sometimes at them, or our colleagues in the business are and we've asked them to help scout)
Talk to confidantes and current or former colleagues/clients about it (we may not know you yet, but we may know them)
If you have genuine search consultants in your network then talk to them (that's what they're there for, if they don't have an interesting position for you they might have some useful information they will share)
Or just approach organisations you might be interested in working for (whether you speak to a Partner or an Secretary they should point you in the right direction).
All the best with the job hunt.