A common soap opera trope would have been that the kiss happened without a witness, and Hope and Thomas kept it secret. But that didn't happen here — as poor Liam witnessed the whole thing. He had voiced his concerns about Thomas to his brother, Wyatt Spencer (Darin Brooks), who reassured him that Hope wouldn't ever be unfaithful. 

This must be another soap amnesia storyline, because it seems that the two have completely forgotten that Hope has cheated several times in the past. When Liam was previously married to Steffy, Hope kissed him. When she and Liam were engaged, she was canoodling with Wyatt. She even married Wyatt when she saw Liam with Steffy and assumed it was something more than a simple conversation. And there was once a point where she vacillated between the brothers, dating them simultaneously.

Yes, when Thomas was rehired to work with Hope, Liam was a bit whiny about the situation, but he had every right to be concerned. However, he didn't expect to be blindsided by Hope's kiss, and quite frankly, didn't need to see it. Liam has been good to her by holding down the fort at home, taking care of the kids and whatever else needed doing, while she spent practically every waking moment with Thomas at work. All the while, her fantasies about Thomas got more intense — to the point where people were starting to notice. Except for Liam.