Robin visiting a Blue Tit box?

Hi all,

We're certain we've seen a Robin popping into our Blue Tit box ( currently empty but being eyed up for shelter by the Blue Tits from last year's brood and Great Tits on and off) 

Does this happen as it seems odd to me? 

We have Robin boxes well hidden and the Tit box is about 2m up on a bare wall so totally opposite to what Robin's normally like. 

We wondered if there were bugs that took its fancy in there but risky if another birds was inside and plenty of ground level bugs for it normally.

Many thank

Steve