2:40am is the time when Subang ATC notified Malaysian Airways that they had lost radar contact with the plane and decided that MH370 was missing.
They actually lost radar contact at 1:21am but there was nothing strange about that as they thought it was heading off to Vietnam.
It was only after Vietnam could not contact the plane and there was a flurry of attempts at contacting it that it was realised that it was missing and MAS should be contacted.
It is perfectly reasonable that in the initial days these facts were sloppily reported by various people as "Radar contact was reported lost at 2:40am".
By coincidence it was discovered several days later that there may have been military radar recordings of the plane elsewhere until about 2:40am. However, this has now been corrected to 2:19am.
Obviously, conspiracy theories can be built on such a co-incidence by those who like to look for such.