Advice much appreciated! BB has always been immaculately litter trained from 13 weeks. He has a big covered litter tray in a discreet corner.
This morning there was a small puddle just outside the entrance. We thought okay, maybe he missed his aim (analogy between young boy cats and small boys?) Then this evening he came in, went into the litter tray and spent some time in there. Then came OUT and with great concentration and taking some time over it, wee'd again just outside the entrance 
Only possible stressful circumstance is that we have had a visitor to stay one night, first time since we had him. She's not especially intimidating and didn't bring a Rottweiler or anything!
He is neutered male, on mix of good quality wet and dry food, eating and drinking fine and not seeming unwell in any other way.
Any suggested courses of action? TIA.