Hotel receipts do not usually indicate the number of guests; however, they do usually state the type of room, and this in turn often (but not always) reveals the type of bed. So there is a theoretical possibility that somebody will wonder why you're staying in a "double room" or "king room" when everyone else is in a "single", although this seems a bit much even for a nosy accounting department. Also, in many better hotels a queen/king bed is standard even for single travellers.
All that said, I would advise you to be upfront with the company and just ask your boss if it's OK to bring along your partner, naturally offering to pay for the flights and any difference in the cost of the hotel. Most companies will not object, and this is, quite literally, a small price to pay compared to the potential reputational damage of the company finding out you did this surreptitiously and thinking that (say) the conference was only an excuse to go on a romantic holiday on the company dime.