Store employees have since confirmed that the 50-year-old singer was there with a woman, but it remains unclear who she is.
'Lack of trust has always been a major factor in their marriage,' a source tells In Touch.
'If Nicole found out Keith lied to her about another woman, she'd be furious.'
Nicole has worked hard in recent months to try and make their marriage stronger, In Touch claims.
'They've had their issues,' a source close to the Hollywood couple spills.
'Nicole feels like she constantly has to go overboard defending their marriage,' the source says of the actress, who often engages in excessive PDA with her husband.
'Their marriage has been slowly crumbling for a long time,' In Touch's source says. 'Nicole has been struggling to keep it together.'
'A scandal like this is the last thing their marriage needs right now.'
Neither Nicole or Keith have responded to In Touch's report.