Suspicious blue tits! What are they doing?

This is my first spring in my new house and my first nest box... and i've been eagerly watching it to see if any birds are planning on moving in... and well, i'm finding it a bit confusing!! There's a blue tit thats been hanging around the nesting box constantly for the past couple of days (at least i think its the same one!).... and quite often another blue tit is nearby, so perhaps the potential partner. I'm not sure what theyre upto though!! The bird seems to perch on the entrance hole and put its head inside the box, then look backout again and look around the garden, and put its head back in.. then out... then in.. and do this repeatedly for quite some time! It's not been taking anything inside the box, so it cant be building a nest, though i have seen a bird hop inside sometimes. Sometimes it takes a break and sits on top of the box pecking, or gets a bit of food from the feeder in the garden.. then carries on with the head in, head out, head in, etc. Any idea whats doing on? Even the pigeon is watching and looking a bit confused!!