It was not immediately clear why the bottle had not been found during an earlier search of the area.
Police are understood to be trying to track down an ex-boyfriend of the victim.
It was reported locally the woman may have been lured to the spot where she was attacked by a man posing as her current partner.
The shocking attack happened about 10.20pm local time on Saturday, near a private hospital in Alvor on the M531-1 road, which is one of the main routes into the town.
Her attacker is believed to have thrown about two litres of acid over her before fleeing.
She was assisted by firefighters before being taken to hospital. She is now in a specialist burns unit at Sao Jose Hospital in Lisbon.
It is not yet clear if the victim lived in Portugal or was on holiday.
Portimao police are said to be hunting an ex-boyfriend, who has not yet been named.
Two officers who helped the woman and were taken to hospital with breathing difficulties after inhaling fumes were today said to be in a stable condition.
Alvor is a seaside resort and fishing village in the Algarve, Portugal's southernmost region, and it has become a popular holiday destination for British tourists in recent years.
It is renowned for having some of the finest beaches of the western Algarve.
The investigation into the acid attack is being led by the Policia Judiciaria in Portimao, which first investigated Madeleine McCann's disappearance under the guidance of controversial ex-police chief Goncalo Amaral.
The attack is the latest in the string of shocking attacks involving acid.
Last month clubbers were doused with a corrosive substance at Mangle nightclub in London. Two people were blinded in one eye and 18 others suffered multiple burns.
Arthur Collins, the former boyfriend of Towie star Fern McCann, has since appeared in court, alongside with another man, Andre Phoenix.
