Gas cylinder & the old lady

I was reading the paper and was shocked. How did it go wrong? How did it happen? I didn't think that this might happen to me. The reason for this happened a couple of days ago.
         It was around 7 o clock in the night.I was planning to go out and a lady knocked at the door.She told she was from Indane gas agency and they were there to check the regulator for each and every house. As a dutiful citizen, i asked for her id card and other details.She showed the details and there was a paper cutting showing that because of an accident , indane has planned to check all regulators in the city.She showed us how to clean the regulator and how to check the life of regulator.She checked the regulator and said it's working fine.
          She told, at any point of time, the gas cylinder won't use 10% of the gas settled at the bottom and she has a pressure valve which when fitted, will suck the residual gas in the cylinder. The price of the instrument was Rs.1000 and it has ISI certifications. Performance improvement and increasing productivity of the cylinder, my corporate brain told. After finishing all the formalities, she gave warranty card and voucher  and left.
     Couple of days after, i was reading the paper and the first page read " Innocent customers cheated by guys posing as Indane distributors" and the "first paragraph" happened.
 The climax of the story happened later and i was left with lot of unanswered. An old lady in the flat didn't believe the indane lady and didn't buy anything. Nevertheless to mention, the indane lady seemed like normal government official, had vouchers,brouchers , bills and had warranty cards for regulators.
How did the old lady in the flat found out that the indane lady was a fraud?

She had only one liner for answer, just like sujatha's dialogue in a Shankar movie.

"WHICH GOVERNMENT OFFICIAL WORK BEYOND 5 O CLOCK"

However much we may be sophisticated , there are some things in which we can't beat the old school of thought guys. Another lesson learnt in another day. A 1000 rs lesson though :)