Peter
A number Claire keeps.
Peter is one of the phone numbers Claire keeps that Tom knows nothing about. He is discreet, thorough, and expensive. Claire has used him twice before, once three years into her marriage to look into one of Tom's business partners. He doesn't need anything explained, and he always answers on the second ring.
The morning after the welcome party, Claire calls him and asks for everything about Quinn Calloway from before her name existed.
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What he did
Eleven years ago, Claire paid Peter to watch Sarah. For six weeks he sat in a parked car on her street and wrote seven reports: who stopped coming by, the days she didn't go out, the kitchen light on in the middle of the night. He pulled her phone records and saw she called one number eighteen times in three days. His last report came four days before she died.
In the present, he digs up Quinn's real name, Rebecca Elizabeth Mercer, and learns her face for Claire. When Quinn tracks him down at a diner six hours away, he already knows who she is. He is about sixty, gray at the temples, in a plain dark jacket, and he looks like no one. He refuses her money and mails her all seven reports. Then he cuts Claire off for good.