These have been left prior to the lot coming under the hammer, and will normally be bid by an auctioneers clerk. If you find that you are the only person in the room bidding, and the price is still rising you are probably against a 'commission' bid. Telephone bids are also becoming more widespread. Again an auction house employee will be on the telephone bidding against you. The other thing you need to be aware of is the 'reserve'.
Some vendors enter an item into auction with a minimum price tag, known as the reserve.
If the item does not reach the reserve, the auctioneer will inform the room, and it is not sold. Sometimes the bidding can go alarmingly quickly. In the rush, it can happen that the auctioneer has accepted a bid from a ficticious bidder, i.e. he has 'Bounced' a bid off the wall. In a large room, this practice is very easy to accomplish. Sadly there is nothing you can do about it without proof.