A woman demanded cash from two Aylesbury shops on Sunday night - but left both empty handed.
Some reports suggested the woman was armed with a knife when she entered the One Stop in Somerville Way at 10.10pm.
She queued up and once at the front demanded cash from the till.
When staff told the woman they could not open the till she left the shop.
Half an hour later a woman, believed to be the same one, went into the Sainsbury’s in Oakfield Road and again demanded the till be opened.
Staff at the shop refused and the woman walked out.
There are no reports suggesting she was carrying a knife at the Sainsbury’s.
The woman is white and was wearing a bandana over her face.
Police are studying CCTV of the incidents.
Officers have asked anyone with information about either incident to call police on 101.