We report COVID-19 multisystemic inflammatory syndrome in an adult patient with an atypical presentation (mild abdominal pain) and a negative (repeated) reverse transcriptase-PCR, in the absence of lung involvement on lung ultrasound. In this case, focused cardiac ultrasound revealed signs of myopericarditis and enabled us to focus on the problem that was putting our patient in a perilous situation, with a quick, non-time-consuming and easy-to-access technique. Serology test was performed and SARS-CoV-2 infection was confirmed more than a week after admission to the coronary unit. As the patient had a general good appearance, the potential implications of missing this diagnosis could have been fatal.