Ok, I will give you the full story of my most recent experience with this.
I was looking into a domain name maybe 18-24 months ago for a project I was considering. I know I checked the availability of the domain a number of times over a period of say 6 months. Then one day I checked it, and the domain name had been taken. I think the trigger was me checking for it multiple times whilst being logged into my Namecheap account. I have no idea whether this is done by the company itself, or by some rogue employees.
Roughly a year later, I started receiving emails asking if I wanted to purchase the domain for I think it was $99 (or it might have been $199). The email did not come directly from Namecheap, nor have any branding related to Namecheap.
But, the only company where I had searched for this domain was Namecheap, and they obviously had my e-mail contact details.
Over the next couple of weeks I received more emails, but they started to reduce the asking price, until I think it came down to $25. Then the emails just stopped. I checked for the domain again, and it was now available, so I bought it (not that I had any immediate plans to push forward with the project, I just wanted the domain!)
So this is what I think is happening. If a registered user searches for a domain name on multiple occassions, they see the user is interested and they register it themselves for a period of 12 months. Next time the user checks the domain name, they get the usual response of the domain not being available, but that you can "make an offer" for it. As they reach the end of the 12 months registration, they start sending emails asking if you want to purchase the domain name. If they don't manage to sell it within the 12 months, they don't bother renwing the registration, and the domain becomes available again.