Check the seams of the mattress, look really carefully, you might find them hiding there.
I can’t go into my story again but I was the family member who reacted to the bed bug bites. We hired a pro, threw out a sofa, box springs, mattresses, books, anything close to where bed bugs had been found. Every piece of cloth in the house washed in hot water & puf in a hot dryer.
All told,the problem cost me around 5K, the thing is the pest control man insisted he gain access to the landlords upstairs apartment, when he did he discovered a half empty bedroom ( right above my living room) he found bed bug markings & carcasses & told us the problem had started upstairs. Landlord had hid bedbug ridden mattress & box spring in the garage a few weeks before we noticed the problem & put up signs trying to keep us out of the garage.
He later admitted to the pest control guy that his son had brought them home & he went & bought bug bombs to get rid of them.
We should have moved right away, not because of the bugs ( which could happen to anyone) but rather due to his lies & attempted deception. In the end he paid the pest control man but nothing more. This was one of the most tiring, tramatic experiences I have ever had.