It's a two-way street, my friend. I only spent about a year in a tower (Dayton, OH) but in that time I was really taken aback at the number of times (probably twice a month) someone came sliding down final with nary a word and touched down. Usually they'd turn off the runway, then you could see them hesitate... and they'd come up on ground frequency.
Once in a while, someone would ask, "uh, tower, did we check in?" My stock answer was, "I didn't hear you so I shot you the green light". Covering your A, there, Cappy.