To get bed bugs they must hitch hike in from your traveling, clothing, furniture, luggage or other items. They do not come in from leaving your door open. They are not invisible. Not every individual will have bites or an itch to notice them. In fact, half our population does not even react to a bed bug bite. While one bug hitch hiking in on a briefcase may seem trivial, they spread quickly. Females can have up to eight eggs ready to hatch. Populations double every two weeks and they only feed on human blood. Ugh!
For folks that that do not react to being bitten, you will only notice when the population has exploded (around 3 months). Then you will see them just as you would ants or roaches. They are small - the approximate size and color of an apple seed and like zippers, edges, corners, mattresses, baseboards, etc.