I've seen that too.
They may be able to favorite you from the snippet of your profile, not sure exactly what's in it. Possibly a picture and maybe your net worth???
Another possibility may be that a premium member has settings allowing them to view you without your being notified. I've heard this but have not verified it.

Haven't seen it from the chick's side but on the guy's side you get a list of chicks.  I think you can favorite from right there without clicking down to their individual page.  
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They are just blasting out favorites.  You can bet they aren't serious contenders and would probably be scammers or seeking validation with no real future commitment in mind.

It helps the bottom line for these sites if chicks contact a lot of people.  So it is probably encouraged and made easy for them.

When you do any kind of a search on SA, your search results show up in one of two formats.  You can switch between formats using the 2 little squares to the left of the "30 per page" box.  Both formats have a small heart shape on them.  If you click the heart, you have just favorited that person without ever viewing their profile.