Some domain companies will buy the domains you searched and try to resell them for a premium. Happened to me just a few months ago. I was using a GoDaddy search since it seemed like a useful way to bulk search some tlds for what I wanted. Looked up a few things that I'm almost 100% sure nobody else was looking for since they were very unique. Didn't buy them that day, but spent a while browsing them using the GoDaddy search tools.

One day later I decided I was ready and went to buy one, but everything I had searched was suddenly only available to buy jacked up with a huge "premium" fee and seemed to be owned by GoDaddy. Never again will I search using their console.

Some registrars have done something called "domain tasting", where they will register a domain when they see interest via their web whois tool. Then they will offer it for a higher price. They were able to register it for five days without paying anything, and then cancel the registration if they didn't manage to sell it.

This has mostly stopped since about 2010 when registrars had to start paying a few cents per transaction. Since the overwhelming majority of those never converted to a sale, it's no longer really economically viable and I don't think it's much of a worry today.