Forgot to mention that one of them may be even using it for night roosting.
For over a year now we have had a Great Tit using our box for night roosting...even in the Summer months he uses it. Unfortunatley he was also there to see off any pairs of BTs that had taken a shine to it. One evening last year him and a perspective BT flew in at the same time, we saw a fight take place via the camera we have in it...the BT left minus a few feathers.
Marmot Avatar by Birdforum Member - Pavlik