The Health and Safety Executive is investigating another possible case of carbon monoxide poisoning on the north coast.
No-one is living in the apartment block on the Portmore Road in Portstewart but there are two shops below.
The owner of one of the shops said two people went to hospital after feeling unwell but were given the all clear.
It is part of the HSENI's ongoing investigations into the deaths of two teenagers in Castlerock a week ago.
Aaron Davidson and Neil McFerran, both 18, died in an apartment last Tuesday.
Their friend, Matthew Gaw, survived.
Earlier on Tuesday police searched premises at the centre of the Health and Safety Executive alert connected with the death of the two teenagers.
They took a number of items away for examination.
On Monday, the HSENI urged anyone who had work done by George Brown, of 'The Calor Shop' in Coleraine, to switch off all gas appliances.
Mr Brown has not commented publicly since the advice was issued on Monday.
'The Calor Shop' is an entirely separate legal entity to Calor Gas (NI) Limited.
The HSENI also revealed a liquid petroleum gas appliance in the flat where the teenagers died was defective.
HSENI said it was continuing to carry out checks in other premises in Castlerock and Coleraine.
The MP for the area, Gregory Campbell said people are very "concerned".
He said it was time for HSE to "clarify" some issues.
"I do think they need to continue to be proactive.
"They need to put personnel in front of cameras, people up to explain what individuals should do because there will be a lot of legitimate concern about various appliances.
"There will also be concerns that can be very easily addressed because people may be getting unduly worried and they don't have to be worried.
"I think some very simple straightforward questions can be answered by the HSE but they need to proactively get out there and answer them," he added.
The HSENI said that the helpline for people concerned over the Castlerock gas leak has so far received almost two hundred calls.
The emergency helpline is 0800 0320 121 and will operate until midnight on Tuesday.
The HSENI has asked that only members of the public with specific emergency safety concerns use the helpline.
In a statement on Monday, the HSENI issued some general advice and said anyone displaying any of the following symptoms - headache, dizziness, nausea, breathlessness, collapse, loss of consciousness, should seek medical help immediately.
They should also contact their local Gas Safe registered engineer and request a full gas safety check.
The HSENI also advise installing an audible carbon monoxide (CO) alarm as a further precautionary measure.
The funeral service for Aaron Davidson took place at The Church of the Ascension, Newtownabbey on Monday.
A funeral service for Neil McFerran was held at Newtownabbey Methodist Mission in Rathcoole on Saturday.
Meanwhile, Enterprise Minister Arlene Foster has said she hoped to meet the finance minister this week to discuss building control regulations which would make carbon monoxide detectors mandatory in all new-builds.
She said "that, of course, would be the case for smoke alarm detectors for new-build properties and I think it should be reflected for carbon monoxide detectors as well".