I have read all of your thread Broken and at first was in agreement with those that said it was a mistake or policy of the hotel or default setting issue that made the reservation a double occupancy, and thought don't go looking for problems that might not exist..
However just a few words of caution from my experience. My ex worked away from home a lot, both in the UK and abroad. His travel office at work always booked his accommodation and he was billed to his company credit card. I never saw the bookings nor the bills. He always rang me each evening without fail often as he was going down to dinner so didn't speak for long just checked in to see if we were ok.
I never once suspected him in all those years but after a while he began locking his phone so I couldn't access it, I didn't question this once. He also began going away on the same night of the week every two weeks or so and stayed in the same hotel. On one occasion a bill for the hotel appeared on our personal credit card statement and when I asked him to get it refunded as a work expense he said he would but no refund ever arrived.
Over time I asked him who he was meeting or dining with and he always gave a plausible reply, usually someone I knew from business or a work colleague, again I never doubted him.
Eventually my suspicions became so great that I asked him outright on more than one occasion if he was having an affair and he denied it every time. I checked his pockets, his brief case, his car, questioned all he did, and got the same don't be silly, I love you, why do I want someone else.
I found out by chance, he dropped his phone, picked it up and it was unlocked, I read texts from OW.
He admitted to a three year affair, I suspect longer, with OW, a work colleague, who travelled with him on many occasions that he told me of.
He even admitted to taking her on holiday when he had told me it was a business trip, and he knew I couldn't go with him due to my own work commitments, so knew I wouldn't question it.
I had such strong gut feelings for 18 months before discovery, but I loved him, trusted him and naively thought he would never do this to me.
I agree with the other posters go cautiously, be certain, the anguish that those feelings of uncertainty, mistrust, suspicion cause are truly painful and hard to live with. I became a paranoid nervous wreck, so much so my DC even noticed. But I stuck with my instincts and eventually found him out, he was very careful, no one knew, they got away with it for so long. Get your evidence, speak with him, if he is a decent person he will be truthful. My ex wasn't decent and showed me no respect at all.
Good luck.