Nest box above barbecue

Recently I notices that there is a small bird box on the front of my Wendy how high up by the roof. Below the box we have our barbecue which we use probably once every 2 weeks or a bit less and we’d presumed that no birds would nest in there. Today I noticed that there is a big nest that fills the whole box that I can see from the ground through the hole, I hadn’t noticed this which is weird cause I have spent lots of tome outside recently, I couldn’t hear anything when I stood by the box and I didn’t see anything go into it. I’m guessing that they bird that was in there has already had its chicks and they’ve fledged? I’m just surprised we didn’t notice it. Could the smoke from the barbecue have hurt the birds and the chicks, or even killed them and that being the reason why I can’t hear anything in there a or see anything going in and out. We had no idea this nest was there it was build so quickly. Also does anyone have any ideas what type of bird could’ve been nesting in the box? The bird box hole is about the size of a sparrow box hole. Could the birds that were in there be okay?

i don’t think you can get into the box so how am I meant to clean it out and look what’s been happening