
How long was the slip mating, you do hear of puppies resulting from them (usualy when you don't want pups) but there has to be enough contact for ejaculation to ahve taken place, so the mating would ahve had to have lasted a minute or more after penetration).
Were you unable to get a tie on subsequent attempts. As a bitch owner I would not have been satisified with a slip mating.
Had you not thought of having your bitch scanned, this should normally be fairly reliable after 28 days from mating.