If you can, make sure customers aren’t taking bed bugs out with them when they leave before a treatment and bringing them back in afterward. This is especially important before a heat treatment only, with no residual insecticide used.
We had an instance where bed bugs were harboring in a laptop. The customer would take the laptop out with him and bring it back in afterward. He would put the laptop on the nightstand every night, and the bed bugs would come out and bite him, and then climb right back in. Finding the source was an extremely time-consuming project.